Atheist: Someone who denies the existence of god.As for me, I prefer the term "Antitheist", which means:
Agnostic: A person who claims that they cannot have true knowledge about the existence of god (but does not deny that god might exist).
Active opposition to the belief in the existence of a GodThere's a great page up on Wikipedia that goes into depth on how antitheism differs from atheism.
The basic gist, however, is that an antitheist is actively opposed to the idea of god and the religions that spring from once people start believing in god. As Wikipedia puts it:
Antitheism may be adopted as a label by those who take the view that theism is destructive.This is where I come out. I don't just lack a belief in god, I actively subscribe to the idea that a belief in god and religion is inherently destructive and causes untold suffering.
Here's Christopher Hitchens:
I'm not even an atheist so much as I am an antitheist; I not only maintain that all religions are versions of the same untruth, but I hold that the influence of churches, and the effect of religious belief, is positively harmful.