What Do Atheists Believe?
I shall quote Baggini's Atheism: A Very Short Introduction, as it gives a very good explanation:
Atheism is in fact extremely simple to define: it is the belief that there is no God or gods... However, many people think that atheists believe there is no God and not morality; or no God and no meaning to life; or again no God and no human goodness... there is nothing to stop atheists believing in morality, a meaning for life, or human goodness. Atheism is only intrinsically negative when it comes to belief about God. It is as capable of a positive view of other aspects of life as any other belief. (emphasis in original)
In An Intelligent Person's Guide to Atheism, Daniel Harbour explains that there are two different ways of viewing the world, both of which require assumptions about that world. In one case, a person might start with some initial assumptions and decide that those are unchanging, that, for instance, there is a creator of the world. In the other, a person will regard everything, whether they started out believing it or added it to their assumptions later, is equally subject to reevaluation as a truth. This second world view changes when new evidence arrives, whereas the first will try to make the new evidence fit into their existing beliefs.
Thus, if you assume that there is a god, and base your worldview on unchangable assumptions, you will maintain that belief as best you can. If you are of the second worldview, and you are presented with lots of evidence that there is no god, you will assume that there isn't, even if you previously thought there was. This also means that the second group is open to the possibility that they are wrong in this belief as well. A non-dogmatic atheist will leave room for the possibility that they are wrong, if the evidence is strong enough.
Harbour says that it is these two worldviews that really divide the atheists from the theists, that we think about things so differently.
Other than the fact that an atheist doesn't believe in a god or gods, his or her beliefs can really be anything. We're a diverse group with different beliefs on other issues. We just have the one in common: we don't believe in gods.