Atheism is the belief that there is no God.
In more detail, Atheism is a school of thought that decrys the concept of God in favor of the idea that we have yet to understand the universe, or even ourselves (as opposed to agnosticism which states man's (perpetual) inability to understand concepts of self, the universe, etc.), and that we may continue on our path to enlightenment through rational thinking and expression (the three backbone's of knowledge and understanding being philosophy, art, and science).
There is a more widely accepted (but not openly spoken of) idea that combines atheism and agnosticism (calling themselves agnostic atheists); empirical agnostics, open to ideas of the supernatural or even God as long as there is observation or proofs [sic] of their existence (In reality, this concept is just a better defined theory of atheism. Perhaps, this separation of labels occurred because of the media's portrayal of atheists as raving, politically correct, fanatics; closed to other ideas or theories).