Simply put, an atheist is someone who doesn't believe in gods. The opposite of an atheist is a theist, someone who believes in at least one god. Or something along those lines.
The precise details of the definition are contested a lot. For example, a lot of people will draw distinctions between "having no god-belief" and "believing in no god." I'm not even going to touch that one. Some people will say that everyone is either an atheist or a theist, but there also lots of people who try to place themselves somewhere in-between. Under this definition, babies are atheists, which is acceptable to some, but not all. Other distinctions include those between "strong" and "weak" atheists, "positive" or "negative" atheists. These are all loaded words, and I'd rather not promote their use.
One thing that all the definitions have in common is that atheism does not imply certainty of God's nonexistence. There's an argument against atheism that goes something like this: "How can atheists be absolutely certain that God doesn't exist if only God can have that knowledge?" Never, ever make this argument. It only shows you have failed atheism 101.
Some people (never people who calls themselves atheists) actually argue that all atheists are wrong, and that atheism actually does mean being certain. They might support this argument with the word's etymology, or historical usage. That's silly. I don't care what the word used to mean, not unless I'm reading old literature. Words change.
So what does the set of all atheists have in common? Aside from not believing in gods, there are and there aren't similarities. Some atheists like to say that most of us have common views like humanism, naturalism, secularism, skepticism, etc. (definitions for these words later!), and this is at least partly the case. Simultaneously, other atheists will talk about the diversity of the atheist group. There's also talk about the "herding cats problem," which refers to atheists' resistance to organization for whatever reasons. So even to me, it's unclear how diverse the group is.