Germanic is a primarily linguistic, and secondarily cultural, term. It refers to a family of related languages that at the time of the Roman empire was spoken in parts of modern Germany, Austria, Poland, Czechoslovakia, and Ukraine, and the people with whom it was associated. There are a variety of theories on the origin of the language and the people, with the two most prominent being a Scandinavian origin, and, more popularly, an origin on the southern coast of the Baltic sea in modern northern Germany and Poland.

Beyond that, you're asking about a very complicated subject referred to as ethnogenesis - essentially, the formation of peoples and ethnic groups. An anthropologist could help you further. As there is absolutely zero remaining written evidence from that period, it's very difficult for a historian to speak to the origins of the Germanic language or culture.

An archaeological "Germanic" culture seems to have emerged in the 8th Century BC, but by the 1st Century BC we see Germanic cultures supplanting Celtic cultures in areas over the Rhine. We see references to Germanic tribes in Caesar's Commentaries I when he discusses the invasion of Ariovistus as a reason for Roman intervention in Gaul. His uncle Marius defeated a folk migration of Cimbri and Teutones. A proto-German language has been reconstructed which may have been similar to the language spoken by the ancient Germans (be warned, it looks very ugly!).

We seem to categorise tribes who were "Germanic" and those who were not with a very Roman perspective. According to Caesar anyone living over the Rhine were Germans and anyone within the Rhine were Celts. He later makes a few exceptions. Archaeological evidence also can disprove these claims - as I stated earlier, Celtic cultures were living in "Germania" but these groups were absorbed or conquered by people of a Germanic culture. Tacitus, who was writing in the 1st Century AD after the conquest of Gaul and the defeat of the Roman forces at the Teutoburg Forest, writes in his "Germania" of a Germania that seems to be pretty much the lands across the Rhine - by this time Gaul was Romanised. This is the image that seems to stick. This does not mean that all the people of Germania were German tribes. Tacitus includes people such as the Fenni, who almost certainly were not Germanic-speaking, in his ethnographic study of Germania and makes the claim that they were Germanic. They were more like Finno-Ugric speakers, like the people of modern Estonia and Finland. Tribes living in Germania therefore probably included a range of different cultural and ethnic groups apart from the Germanic but thanks to Roman writers we tend to lump them together as broadly "Germanic". This categorisation may have continued into the later period of the Roman Empire, and Harry Sidebottom claims that this may have been due to the Romans seeing this new "Germanic" threat as a continuation of the old Germanic threat posed to the early empire. Sorry, hope that wasn't too confusing!

Regarding the major tribes, it is perhaps a bit difficult to go into too much depth here. From the early period, significant tribes include the Cherusci, Suebi, Cimbri and Teutones. Each played a significant role in early Roman history. The "Germania" has more information about the history of many tribes. Significant tribes from the later period include the Goths, Franks and Vandals, who helped bring down the Roman Empire. Jordanes wrote a mostly inaccurate history of the Goths but there is loads of stuff about later Germanic tribes, especially compared to early Germanic tribes where we seem to know next to nothing.

Tacitus' Germania: http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3atext%3a1999.02.0083 Caesar's Commentaries: http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.02.0001

Hoped that somewhat helped!