The Germanic culture has managed to keep its own identity due to its isolation in sometimes impenetrable areas, however, it has influenced and has been influenced by other cultures troughout history so it also contains aspects from other cultures which have been picked up via personal contacts, trade, and even war.
A union of kin:
Before the culture that we now call "Germanic" came into existence northern Europe was inhabited by several related cultures, this cultures were indirect descendants of the native Funnelbeaker culture (3400BC to 2850BC) but they also had many Indo-European influences in their culture and language.
The Indo-European migrations took place from approxymately 4000BC to 1500BC though in northern Europe the Indo-European influences started around 2500BC, most of the Pre-Germanic cultures had adopted aspects of the Indo-European culture and language which resulted in a combination of the native northern European culture and Indo-European influences.
The Germanic culture descended from this cultures so those Indo-European influences can also be found back in the Germanic culture, though the Indo-European share in the Germanic culture should not be overestimated since most of the Germanic culture probably indirectly descended from the native Pre-Indo-European culture; the biggest Indo-European influence on the Germanic culture was its language; somewhere around 60% of the Germanic language can be traced back to an Indo-European origin while 40% is probably derived from the native northern European language, though personally I believe this percentage is a bit exaggerated because I have done many comparisons and have not been able to find more than 10% to 40% of Indo-European influences in my native language (Dutch).
The first Germanic cultural group was the Jastorf culture that inhabited northern Germany, they started using iron in 500BC which gave them an advantage over the other northern European cultures, eventually the Jastorf (Germanic) culture became the dominant culture in northern Europe which resulted in a cultural union between the people of northern Europe and the birth of the new Germanic identity.
More about the historical origins of the Germanic culture can be found in here.
Eastern influences:
Another very interesting aspect of Germanic culture is that it has some similarities with the early Asian cultures and that the Germanic religion posessed some aspects of the Buddist and Hindu cultures, the explanation for this must be searched in a common Indo-European origin of this similarities.
As an example; Germanic mythology is one of the few European mythologies that has legends about dragons, which are very common in Asia.
Another aspect is the belief in a world tree (the ash tree Yggdrasil) which is also present in a lot of Asian religions.
Germanic art also contained influences from Asian steppe art which they may have taken over from the Iranian nomads (Scythians, Sarmatians, etc.) that inhabited the Russian steppes, or vice versa of course, though a more plausible explanation is that this peoples had influenced eachother and/or that some of their art styles were derived from Indo-European roots.
In a later time during the Great Migrations the eastern Germans also had contacts with the nomads in eastern Europe, something that can still be seen in the Gothic art from that time.
Differences between northern, western, and eastern Germanic culture:
Due to geographical separation and local changes the Proto-Germanic culture was eventually divided into three main groups; north, west, and east, this differences increased as time passed by though the differences between this three groups remained so small that they could still be recognized as belonging to one group, even in the Middle Ages the English, Dutch, Germans, Scandinavians, and other peoples of Germanic origin could easily communicate with eachother without a translator.
Even nowadays the differences aren't that big; if you use a Nether-Saxon dialect in Austria you can easily be understood by the German-speaking population there (experienced that myself) and Norwegians, Swedes, and Danes can have a chat while all speaking their own language.
Nowadays there are people who do not consider the ancient Germans to be a single cultural group and have separated them into several sub-groups which they claim to have been separate cultures, this groups are;
"Vikings" (Germans in Scandinavia)
"Teutons" (Germans in mainland Europe, including the western AND eastern Germanic peoples like the Goths)
"Anglo-Saxons" (Germans in Great-Britain)
Most scholars disagree with this unprofessional classification and they are absolutely right, this separation of our common Germanic ancestors into various local groups is nothing more than an attempt to cover up our common origin; after world war 2 our Germanic ancestors became a taboo subject which has caused many people to start using different names for them (like "Goths", "Teutons", "Saxons", "Vikings", etc.) or simply claiming that their ancestors were so different from the Germanic main group that they were a separate people, another problem (at least in the English language) is that the word "German" does not only refer to our Germanic ancestors but also to the modern inhabitants of Germany, a nation where even today many people do not want to be associated with even though most of the modern Germans were born after the war.
It can even be said that the above classification has only been invented by people who wanted to deny their kinship with the modern Germans (or other people they happened to hate) or to be "politically correct", and since this site does not have any personal agenda and is not politically orientated I am not going to support this dubious classification on this site.
A better classification that is also accepted by serious researchers is the separation into northern-, western-, and eastern Germanic:
Northern Germanic: the Germans in Scandinavia.
Western Germanic: the Germans in the Low countries (Benelux), Germany, northern France, the Alps, and Great-Britain.
Eastern Germanic: the Germans in Poland, Czechia, and other parts of eastern Europe (this group is now extinct).
This three groups all had a single Proto-Germanic origin and although there may be some differences between this local cultural varieties they can all be considered related cultures that stood in close contact with eachother and had a single identity.
I shall now list some of the minor differences that existed between this three groups;
Northern Germanic culture: the northern Germanic culture eventually became the most homogenous of the three groups, the main reason for this was probably their isolated position in the Scandinavian peninsula as well as the long time it managed to prevent foreign influences like the Christianization.
The northern Germanic peoples, who are commonly known as "Vikings" though a more correct name would be "Northern Germans" or "Scandinavians", had a very rich culture of which most has fortunately been preserved in later times.
The northern Germanic branch was also the last one that got converted to Christianity which allowed them to continue developing their culture without foreign influences while the other two branches were Christianized much sooner and never got that chance.
The Scandinavians were a real sea-faring people; their food mainly consisted of fish and they developed ships that were superiour to any other in the world in that time.
To them, water was not an obstacle but more like a highway; they used seas and rivers to loot, trade, and discover; they even discovered America before Columbus did.
Western Germanic culture: unlike the northern Germans the western Germans had lots of dangerous neighbours like the Sarmatians, Slavs, Celts, Dacians, and Romans, due to the constant conflicts that arose with this peoples the western Germanic culture had a much more militaristic nature than the northern group; unlike in our modern times, war was seen as something positive by the western Germans because it offered lands, wealth, slaves, and most important to them; glory and honour.
As an interesting side note; the English word "war" and the French word "guerre" are both derived from western Germanic "werra" or "gerra", which means "war" or "confusion".
It goes too far to portray the western Germans as war-mongerers, but war did held much importance to them, one of the main reasons for their warlike culture is undoubtly the Roman presence near their borders; only a people that lived, ate, drank, and breathed war would be able to hold out against the mighty Roman legions.
Although the western Germans had a very warlike spirit it can not be said that they were undeveloped barbarians; lots of proof against this has been found; beautifully decorated weapons and jewellry, runic writings, heroic poems, rich graves filled with personal belongings for the afterlife, and many other signs of civilization.
An interesting description of the western Germanic culture can be found in Tacitus' "Germania".
Eastern Germanic culture: the most famous of the eastern Germanic tribes are of course the Goths, but also the Burgundians and Vandals were of eastern Germanic origin, from this last tribe the word "vandalism" is derived because they once severely plundered Rome and trashed half the city, though it is believed that this events were exaggerated by later writers.
The eastern Germans left some beautiful writings; both the Dietrich/Thidrek saga and the Beowulf saga are of Gothic origin.
Gothic is also one of the oldest Germanic languages of which written records exist (Codex Argenteus or Wulfila bible).
The eastern Germans were real travellers, their place of origin is believed to have been Scandinavia and/or the Vistula area but as soon as they experienced overpopulation or famine they formed a subgroup that would then migrate to another area to ensure the survival of both groups, this tactic was also used by the other Germanic tribes but the Eastern group is especially known for this; they travelled for long distances and from Scandinavia(?) they first migrated into eastern Europe, then to the Balkans, and after that to as far as France and Spain, which was a long way in a time without cars or airplanes.
Celtic influences on the western Germanic tribes:
During its early beginning the Germanic culture was influenced by the Celtic Hallstatt culture, eventually the Germanic identity became stronger in northern Europe which caused this influences to diminish, though Celtic influences continued in the border areas (like the Netherlands and southern Germany), where the Germans even adopted some minor Celtic godesses like the horse godess Epona, Dutch archeologists have also discovered some Celtic coins in the Netherlands that the Germans have probably used to trade with the Celts since they did not use any money themselves in that area.
Some people claim that Germanic art has been heavily influenced by Celtic art but that is total nonsense; although they may have exchanged some artistical inspirations with the Celts the Germans definitely had their own art styles that were already used in earlier times, a more likely explanation is that some aspects of the Germanic and Celtic art have been derived from their common Indo-European roots and native (Pre-Indo-European) art forms.
It has even been claimed that there was a Celtic presence in Denmark and some books about the Celts display a map of Europe with a Celtic "culture island" in southern Denmark; this theory is purely based on a handfull of Celtic findings in that area like the Gundestrup chalice, a beautifully decorated Celtic pot that probably ended up there as warbooty or via trade, personally I find this theory highly questionable but some people seem to believe in it.
Germanic influences on their neighbours:
The Germans on their turn also influenced other peoples, the culture of the early Slavonic and Baltic peoples contained a lot of Germanic influences, for instance the Baltic thundergod Perkunas had a lot in common with the Germanic god Thunaraz, the Slavonic stormgod Perunu also had lots of resemblances with Thunaraz, although the various storm- and thunder gods in early European religions have probably been derived from a common (Indo-European or Pre-Indo-European) origin there are still lots of typically Germanic influences in the religions of the peoples that lived near them, especially the early Baltic culture and religion had many things in common with their Germanic counterparts.
There have also been discoveries of Turkish and Bulgarian runic writings which looked much like the Germanic runes and can still not be properly explained.
In a later period the Vikings founded a small empire in Russia which also greatly influenced the Slavs and even the nearby Byzantine empire.
Roman influences on the western Germanic tribes:
The Romans never succeeded in conquering Germania though they did temporarily occupied parts of the Germanic lands, my country the Netherlands is a good example of an area where the cultures of the Germans and Romans melted together.
There was a heavy hostility between the Romans and the Germans and in the 1st century Aulus Hirtius even wrote that "the Germans never refuse to help someone fight against the Romans", despite this the Germans in Germania Inferior (the Roman province that contained parts of the low countries and northern France) also desired the luxuries that the Romans had brought there and were influenced by their occupiers.
They started trading with the Romans and took over some of their customs, the Romans on their turn were impressed by the tall, fierce Germanic warriors and recruited a lot of them into the Roman army, they also traded their goods for slaves, hides, and amber that the Germanic tribes collected at the coastal areas of the Northsea.
The Gallo-Roman occupiers also took over some Germanic customs and they even erected sanctuaries for Germanic gods; in the Netherlands Roman altars have been found that were dedicated to Germanic godesses like Nehalennia and Hurstrge, on their turn the Germans also took over some Celtic and Roman gods.
The Roman influence on the Germanic culture as a whole was rather small since they only occupied some Germanic border territories, though their influence sometimes reached further over the border.
Most of the "Roman" influences in modern countries with a Germanic background have been added later during the Middle Ages, the Renaissance period, and the Napoleontic age in which lots of French and Latin words and influences were introduced because they were considered chic in that time, many European nobles even started to speak French amongst eachother instead of their native language because French sounded so very "classy".
Science and the medical world also introduced a lot of Latin influences, think about plant names, animal names, medicine names, etc. but this also occured in a much later period.
Christian influences on Germanic culture and vice versa:
Christianity have had lots of influences on the Germanic culture, it started under the Romans but in a later period, after the Great Migrations, Christianity became almost destructive to Germanic culture and religion; polytheďsm was not allowed, so the Germans were not allowed to believe in their gods anymore, women were seen as manipulating liars, so the high position of women in Germanic society deteriorated, nature was not seen as something holy by the Christians but rather as something that should be overcome, so holy trees dedicated to Germanic gods were chopped down and holy forests, cliffs, and even mountains were "purified" from their "evil" forces by priests who sprinkled holy water all over the place, beautiful ancient statues, temples, writings, altars, etc. were destroyed, (a good example is the destruction of the Irminsul, a holy pillar of the Saxons) Gođi's (Germanic priests) and Seiđkona's (women who practiced magic) were seen as witches or devilworshippers and were often burned alive at the stake, actually the Germans had two choices; converting to Christianity, or death.
The early Christians did everything in their power to destroy the heathen religion, and they mostly succeeded in it.
Please don't get me wrong; I'm speaking about Medieval Christianity here, not the modern version, it is just a hard fact that the Christian religion is responsible for destroying some of the oldest religions in the world, not only the Germanic religion but also many others like for instance the Celtic, Slavonic and Roman religions and the beautiful ancient cultures of south- and central America.
It is true that some heathens voluntarily converted, but most of them only did it at swordpoint or for economical reasons; for instance, some Germanic kings converted because they were offered lots of money, and Iceland was converted by high economical pressure and boycot.
The conversion did not go without struggle however, when the Anglo-Saxon bishop Bonifatius (whose real name was Wynfrith) tried to convert the Frisians in 754AD, he chopped down a holy tree dedicated to Wodan to show the heathens that their god did not protect his tree and thus did not exist; the Frisians became so angry that Bonifatius had to flee being chased by the entire village, at the city of Dokkum Bonifatius was caught and as a last resort he held the bible over his head and asked his god to protect him; nothing happened and he was killed by the Frisians who probably laughed over this irony.
Germanic influences in our modern culture:
Some parts of the "pagan" Germanic culture could not be destroyed because they were so deeply rooted that everybody refused to abandon them, so they were integrated into Christian holidays; during Easter we still practice lots of Germanic rituals like painting eggs and lighting Easter fires, and Christmas has the Christmas tree, and did you really think Santa Claus has something to do with the birth of Jesus?
The Dutch once founded a colony in America that they named Nieuw-Amsterdam, and on December the 5th they celebrated "Sinterklaas" (Saint Nicolaas) there, a Christianized version of the Germanic Jól celebrations, later Nieuw-Amsterdam was turned over to the English who named it New-York, and the English also took over the word "Sinterklaas" and changed it into "Santa Claus", which was easier to pronounce for them, the English also had their own (non-Christianized!) version of the Germanic Jól celebrations that featured an old man with a grey beard and a green cloak (alternative version of Wodan) who had little elf-like helpers (Alfen), this English Christmas celebration was mixed with the Dutch Sinterklaas celebration which resulted in the new name "Santa Claus", this beautiful piece of folklore continued to modern times until it was commercially exploited by a cola company that turned the old grey man into a fat man with a white beard, and of course his green cloak became an outfit in the red-and-white colours of the company.
Although many aspects of the Germanic culture (especially the religious parts) were lost there is still much of it left and there are many modern countries whose heritage directly descends from their Germanic ancestors, but unfortunately our Germanic heritage is nowadays taboo in most countries because of its abuse by the nazi's and the unjust association with extremist ideologies that followed on that; however, it is still there: a lot of our holidays and customs are of Germanic origin like giving presents at Christmas and birthdays.
The Christmas tree is also a remnant of the Germanic culture; trees were holy to the Germans and during the celebrations of the Wintersolstice (Jól or Yule) they used to decorate a tree outside and put offerings to the gods at its foot, later this custom was adapted by cutting the tree down and bringing it into the house.
Other customs of Germanic origin are the lights in the Christmas tree, mistletoe, singing songs around the fire, being together with family, I once had a little quarrel with a Christian man after I had claimed that most of the cozy and enjoyable things we do at Christmas are of heathen origin...
In the Netherlands and Belgium the people always celebrate "Sinterklaas" at December the 5th.
Sinterklaas is a saint dressed in a red robe bringing presents to the children together with his funny black helpers ("zwarte Pieten;" or black Petes) who toss "pepernoten" (peppernuts: some sort of small cookies) through the room for the children to collect, this festival has much in common with Santa Claus; both Sinterklaas and Santa Claus are based on the Germanic "Wild Hunter" myth in which Wodan rode through the skies and over the rooftops with his eightlegged horse Sleipnir in the Wild Hunt, a happening that was believed to take place in the time we now celebrate Christmas and new year.
I don't know if this is also a custom in other European countries but in the Netherlands it is tradition to decorate a bundle of twigs ("Paastak" or Easter branch) and hang it near the fireplace at Easter, which is definitely a trace of the ancient fertility rituals that were held in those times.
Actually, the English word "Easter" comes from "Eostre", which was the Anglo-Saxon name for Ostara, a nature godess to whom a holiday was dedicated that was held at the same time as Easter, the German word "Ostern" is also derived from her.
Eggs, rabbits, and chickens were also holy to Ostara, and guess what the symbols of Easter are? right! eggs, rabbits, and chickens.
In the Netherlands children still hold processions during Easter in which they carry decorated sticks with a piece of bread in the form of a chicken on the tip, when I was a little boy I also took part in those processions and most of the rituals we performed were of heathen origin, I wouldn't be surprised if those processions were once held in honour of Ostara, the godess of Spring.
The Easter bunny still brings eggs to the children and the painting of eggs is also a remnant of the ancient Spring celebrations; in those times the eggs were painted and hidden in the field to fertilize the Earth (the egg was associated with fertility and life since living creatures were born out of them in the form of chicks).
Carnival; dressing up in costumes, drinking and partying are all parts of Germanic new year and fertility rites, and why do we light huge bonfires at new year? and why do we make noise with fireworks, horns, or drums at new year? and why do people dance around Maypoles in spring? all of them are surviving remnants of the ancient culture of our ancestors.