I just started reading Atheism: The Case Against God, and I couldn't have discovered this classic at a better time. I first heard of George H. Smith's book back when I was in college, but I had forgotten about it until recently. It turns out that it is exactly what I needed right now.
The book reminded me of precisely what atheism is and what it is not. Simply put, atheism refers the absence of theistic belief. That's it. It doesn't mean anything else. Atheism is not a religion, a philosophy, a worldview, or anything of the kind. It is not the conviction that there are no gods or other supernatural entities. Rather, it is the absence of a belief in god(s). We can attempt to derive subcategories of atheism (e.g., positive, strong, radical, etc.), but these are neither necessary nor particularly useful. Atheism is nothing more than the lack of belief in gods.
As Smith points out, this trivial-sounding definition is actually quite important because it reminds us that the burden of proof rests solely on the theist. While we must provide evidence to support our positively asserted beliefs (e.g., Christianity is destructive, theism is correlated with intolerant views, etc.), it is nonsensical to expect evidence for atheism. If the theist fails to make a reasonable case for the claim that gods exist, atheism is the only sensible position. This is how knowledge works - the group advocating belief in something bears the burden of proof. Nobody expects you to prove that you do not have a fairy godmother, but if you claim that you do, we all (including Christians) expect evidence. Belief without evidence is irrational, to say the least.
When the believer is denied his/her first choice of argument (i.e., asking us to prove that he/she is wrong), only one argument remains. This may take many forms initially but can ultimately be reduced to some variation on "I believe because it makes me feel good to believe." I can think of no other scenarios where we (Christians included) would make such a statement and expect to be taken seriously.
If I tell a room filled with Christian medical professionals that I have a cure for cancer, they would request evidence to support my claim. If I could provide none but insisted that maintaining this belief improves my self-concept, grants me a sense of purpose and efficacy, etc., they would not hesitate to laugh in my face. Their response would be along the lines of, "Just because thinking you cured cancer might feel nice doesn't mean that you actually cured cancer." They would request evidence for my claim, and in the absence of any such evidence, they would wisely reject it.