The Col: I actually don't fully know the difference either. But and agnostic is one who admits that he doesn't really know (perhaps due to lack of evidence or other factors). And anti-theist is somewhat of a activist on proving (through arguments) the non-existence of God (whom they don't believe exists). But they are active against the idea of God. An atheist seems to me one what believes they know God doesn't exist. They reject all arguments given for the existence of God as insufficient.

The only atheist I ever appreciated and like to hear his views is Hitchens. At least in the interviews I saw, he was always eloquent, respectful, and knew his stuff.