Julian Baggini explains in Atheism: A Very Short Introduction the meaning of an atheist's commitment to naturalism: 'What most atheists do believe is that although there is only one kind of stuff in the universe and it is physical, out of this stuff come minds, beauty, emotions, moral values - in short the full gamut of phenomena that gives richness to human life.'
I picked this up in The God Delusion, by Richard Dawkins. (You can read Chapter One for free. I'm still waiting for some one to give me a copy for my birthday.)
I guess I'm more of a heretic than an atheist, because I can't accept that all things are physical. Also, I believe in certain things, like the power of rituals and animal sacrifice.