There are several kinds of definition of what an atheist is, used by different groups to give them a debating advantage.

Probably the worst is that atheism is a religion of no God.  That is as far as I can tell absurd.  Religion, except for some pretentious philosophies, is all about gods, and these philosophies have the equivalent of gods, just not called that.

In fact, to most people atheism is not even a belief. but more just an opinion, albeit usually a strongly held opinion.  The way I would put it is that I am as sure that there is no God as I can be sure of anything, the world being such that one is never absolutely sure about anything.

Of course those who are truly on the fence, who doubt there is a God but have strong doubts about that doubt, who are usually called agnostics.  The agnostic is on the fence; the atheist may admit a remote possibility but is pretty damn sure.  The atheist sees the fence but is nowhere near it.

However, agnostics can be classed as a type of atheist, since the usual off-the-cuff definition of atheist is one who does not believe in God, and the agnostic does not believe as such.

Back to the bad definition of atheism as a religion.  The reason religionists like that definition is then they can say it is just a belief, like other beliefs, and one is not more valid than another, we just choose.  The thing is these people believe because they want to believe, and maybe even cannot imagine not believing, generally because they were taught to believe in childhood.  Hence the exact flavor of one's religion is highly correlated to where and when and how one was raised. 

The atheist on the other hand takes the view that although one cannot prove a negative, if one wants to assert something important one must have proof, or at least lots and lots of evidence.  It then becomes a matter of looking at the evidence present that God exists and coming to the conclusion that sufficient evidence is missing.  The heavens do not declare the glory of God, nor does nature.  There are no asterisms spelling out the Tetragrammaton.

Absence of evidence in favor of an important assertion logically requires a negative conclusion.  An honest person does not accept things because one likes them or because one wants to or because one wants to go to Heaven or because one's parents and culture believes it.  The only honest way to think something is true is because one is persuaded by an honest investigation of the arguments, having paid attention to all sides of the issue.

Would you believe it, so far I have not tried to define "God," usually the first question in this sort of discussion.  There is God and there are gods.  To me the former, to be "God," omni-various things, such as omnipotent and omniscient are implied.  Omnipresent or omnibenificient would count but aren't necessary.  Even a transcendent, spiritual being would be something like an angel or superman, not God.

This leads to the self-referential contradictions we have known about since the Middle Ages, having to do with whether God can make a rock so big he can't move it or whether we can really have free will, and not just an illusion, if God knows all the future (these are two different issues and theists have differing approaches, but I want to keep this fairly simple).

The way the theist tends to deal with this sort of thing is simply by saying that God can do anything except something impossible for God to do, one of the great cop-outs of all time.  I can do anything except something impossible for me to do, so am I God?