Germanic gave English, German, Dutch, Frisian, and the Scandinavian languages, and it now has more speakers than any other branch of Indo-European except Indo-Iranian, almost entirely because of English. Its earliest substantial document is Wulfila's fourth-century Gothic Bible translation, which is an accident of exactly the kind historical linguistics depends on: an entire archaic language survives principally because a bishop translated scripture into it. Runic inscriptions from the second century onwards give scattered earlier material, mostly short.
Old English, Old Norse, Old High German and Old Saxon follow from the seventh century onwards, all of them written by Christians. The branch is defined by a set of sound changes, the first of which was described by Jacob Grimm and bears his name, and which turned Indo-European p into f and t into th, among others: the reason English father corresponds to Latin pater is Grimm's Law. Proto-Germanic itself is reconstructed rather than attested, and sits in the atlas as the ancestor this branch is derived from.
The branch divides conventionally into three: East Germanic, of which Gothic is the only substantially recorded member and which is extinct; North Germanic, which gave the Scandinavian languages; and West Germanic, which gave English, German, Dutch and Frisian. The loss of the entire eastern division is why Gothic, a single translated Bible, matters so much to the reconstruction.