As others have said, Atheists don't believe in any gods. Most Atheists would argue that Atheism is not a religion, because it is an unorganised belief system (in the sense that there are no specific doctrines, no designated leaders and too wide a variety of beliefs etc).
To further illustrate, check out this pick from the Agnosticism spot:
link There are certain "levels" of Atheism/Agnosticism that madami accurately describe individual's beliefs. I suggest also checking out the Agnosticism spot:
link Agnosticism is a varied belief system as well, but it focuses on the belief that people do not know and/or cannot know the true nature of the universe/the existence of gods etc.
Feel free to visit the Atheist club on Fanpop:
link You can take a look around, but be warned that there are anti-religion sentiments there. You will probably also see a number of instances where religious people have come to the club and attack our beliefs. A lot of those conversations are quite offensive to all parties. But you will also find some great conversations between religios people and Atheists, as well as conversations between Atheists that ipakita that Atheism isn't as simple a belief system as the dictionary definitions imply. If you have any questions, I suggest that you check out this forum:
link You may find some sagot there, and if not, you can ask us! There are lots of great forums there for anyone to check out. All that we ask is that you treat us with respect. Treat us in the same way you would want to be treated.
I don't mean to sound like I'm advertising or anything. I just think that this is a great question, and it is great that you didn't just accept those simple definitions of Atheism that you found.