Atheism is, simply put and by definition, the belief that there is no God nor divine presence. Some atheists define it as a lack of belief in a God or deity, while others discover that the proper "label" for them would actually be agnostic, rather than atheist.
"Believing there is no God gives me more room for belief in family, people, love, truth, beauty, sex, Jell-o, and all the other things I can prove and that make this life the best life I will ever have."
However, perhaps this excerpt best illustrates why there are over a billion atheists in the world:
Somewhere in the world a man has abducted a little girl. Soon he will rape, torture and kill her. If an atrocity of this kind is not occurring at precisely this moment, it will happen in a few hours, or days at most. Such is the confidence we can draw from the statistical laws that govern the lives of 6 billion human beings. The same statistics also suggest that this girl’s parents believe at this very moment that an all-powerful and all-loving God is watching over them and their family. Are they right to believe this? Is it good that they believe this?
The entirety of atheism is contained in this response. Atheism is not a philosophy; it is not even a view of the world; it is simply a refusal to deny the obvious. Unfortunately, we live in a world in which the obvious is overlooked as a matter of principle. The obvious must be observed and re-observed and argued for. This is a thankless job. It carries with it an aura of petulance and insensitivity. It is, moreover, a job that the atheist does not want.
From An Atheist Manifesto by Sam Harris