Wiki covers the political background well enough:
http://en.wikipedia.org/wiki/Gallic_Wars
South-eastern Gaul (Transalpine Gaul) was already a Roman province before the war:
http://en.wikipedia.org/wiki/Transalpine_Gaul
The Arveni were a Roman client state (Protectorate, in EB terminology) and the Aedui were Roman allies.
Remember, an alliance, in Roman terms, doesn't mean an equal partnership - it means Rome is the big guy (like the 20th Century USSR in the Warsaw Pact), the ally is the little guy who follows 'suggestions' without question (like 20th Century Poland or Rumania in the Warsaw Pact. And like Poland and Rumania, most of the Gallic tribes were former enemies of Rome, and weren't exactly overjoyed to be under Roman hegemony, even though they hadn't been made into Roman provinces (yet).