Invading German Tribes
In terms of government, all the Germans brought with them was a system that formed the basis for feudalism. That is, they had a small number of families that had held tribal leadership positions for many generations and were considered "nobles." These noble families also served as the priestly class, and some claimed descent from the pagan gods to reinforce their hereditary authority. At the time Rome fell, most adult German males were expected to turn out with some weapons and serve in the tribal army. Over the next few centuries, a large minority of the population held sufficient wealth to equip themselves with more impressive weapons (especially a horse and armor). These became the knights. A large minority of the tribal population were semi-free, serfs or slaves.
Once the Germans had ceased their wandering and settled down in Roman territory, the warriors held sufficient land and serfs to support them and a few followers as warriors. The nobles and kings (there were many kingdoms) presided over the knights (or ritteren, riders in German), who in turn kept the serfs and slaves in line. There was also a class of artisans, including blacksmiths, jewelers, and the like, who used their skills to maintain their position as unarmed free men. The Germans had already adopted many elements of the Roman "estate" system. This was a pattern that emerged early in Roman history. Wealthy land owners would create what were, in effect, company towns, where the estate owner controlled (and often owned) everything. The land was worked by tenent farmers and, increasingly, slaves. Over the centuries, these large estates became more common. Because the Romans offered the prospect of freedom for good performance to keep their slaves from rebelling, the slave population declined. But the ex-slaves increasingly became landless serfs, working fields rented from their former master. If this sounds remarkably similar to the feudal "Manor System ," it is. In fact, it's nearly identical. By the 3rd century, the Roman emperor had these estate owners administering the law on their lands and recruiting troops from among their serfs to supply (or supplement) the imperial army. This was feudalism, the Germans found it compatible with their own attitudes and adopted the system. What we think of as feudalism did not develop during the "Dark Ages ," but was largely a carryover from Roman practice. You could say that the Romans invented feudalism and the Germans embellished it and kept it going through the Medieval period.
The "Wandering of the Nations," as the Germans call this period of mass population movement, actually went on for over four hundred years. The first movements, in the 2nd century AD, were to the east, against the Slavic tribes on the Russian steppes. This was of little matter to the Romans. But all the Germans (in what is now Germany) were prospering because of new technology coming in from Roman lands and the population pressure continued to cause tribes to simply pick up and move off towards greener pastures. After being held at bay by the Roman armies for nearly five centuries, the Germans finally overcame the imperial frontier garrisons in the 4th century and began moving into Roman lands. Spreading as far as North Africa, the wandering didn't end until the early 6th century. Most of these tribal movements consisted of fewer than 50,000 men, women, and children, including slaves. About twenty percent of these were warriors and they generally outfought everyone they faced. The tribes that settled in Roman territory was vastly outnumbered by the local populations. These Germans were absorbed culturally, leaving behind blonde hair, blue eyes, a variety of Germanic customs and little else. France, Spain, and Italy were "germanized" in this fashion and the dialects of Latin spoken in those regions became French, Spanish, and Italian over the centuries, under the influence of various different German languages working on their native Latin.
The German invaders were also eventually converted to Christianity. As the new overlords of Roman lands, they were eager to take advantage of the advanced Roman civilization they had long admired from afar. The Christian clergy were equally quick to cooperate with their new overlords and to "civilize" them as soon as possible. It took about a century longer to Christianize the Germans still in Germany, and even longer to transfer aspects of Roman civilization into the vast territories east of the Rhine river.
Karl der Grosse (to his German speaking descendents) , or Charlemagne (to his Latinized descendents) was the German king who best demonstrated the effect of Roman culture and Christianity on the Germans. This 8th century German noble unified the German tribes (well, most of them) for the first time in their history. He did so with the help of Christian missionaries (for many Germans were still pagans) and skilled clerics trained in the ways of Roman administration.