In Quake Wars, you can play through a series of single-player campaigns as either the GDF or the Strogg, or connect to the Internet to play the same campaigns against other players online.
Enemy Territory: Quake Wars is not only the prequel to Quake II, but it is also a continuation of id's other Enemy Territory game, Wolfenstein: Enemy Territory, except of course, in the Quake universe. This means it has all of the features that made Wolfenstein's version as good as it was, but also quite a few added features to make the game that much better.