Becoming an Atheist means you do not believe in deities/gods in any form and completely reject the idea of such notions.
However, being an Atheist isn't necessarily a complete rejection of religion / world-views. Many atheists subscribe to philosophies such as humanism (big in the post WWII era), rationalism and naturalism. Interestingly enough, some factions of Buddhism have also been labeled Atheist as they do not believe in gods/supernatural.
Atheism differs from Agnosticism in that it is an outright disbelief whereas Agnostics simply believe there is no scientific proof that such deities exist, and hence don't.
Atheists generally stand for and advocate rationalism, non-religious rights, offer science as an alternative to God-based religion. I know here in Australia, Atheist groups advocate on removing the tax-free status of the church.
So as you can see, it's not a simple thing, and includes many different people!