There's a joke I once heard that goes something like this: After a lot of agonizing over the effect it would have on his parents, Patrick finally told them that the reason he no longer went to church was because he didn't believe in God. "Is that all it is then?" his mother replied. "Well at least you're not an atheist!"
It may sound funny but that actually illustrates one of the problems atheists run into. When someone asks me the meaning of a word, I usually send them straight to a dictionary. That, as it turns out, is not always safe when the word has as much emotional freight attached to it as "atheism" does.
Take, for example, Merriam-Webster's Online Dictionary. I love its definition of atheism because it is not only informative but inadvertently funny. Take the very first definition it offers for the word:
1 archaic : ungodliness, wickedness
That view of atheism is hardly archaic since a lot of people still hold it but thank you, Merriam-Webster, for trying to relegate it to the past! They do a little better on their second and third tries:
2 a: a disbelief in the existence of deity
2 b: the doctrine that there is no deity
Let's look at "2 b" first. This is what most people think all atheists believe. It's sometimes called hard atheism and is one of the tenets of philosophies like Communism. It's a dogmatic viewpoint and in its way, kin to religion. That's because the baldly-stated assertion that there are no gods is as unprovable in absolute terms as its opposite, and therefore is also faith-based. Most atheists hold their nose at the first whiff of absolutism and shy away from viewpoints like this one.
"2 a" is often called soft atheism to distinguish it from the dogmatic version defined above. There is a world of difference between saying "I don't believe in gods" and "there are no gods." Soft atheism acknowledges that the universe is so large that it is impossible to prove absolutely that some kind of deity or deities aren't hiding in some dusty corner of it (of course, this concession depends on how you define "god." Some versions include attributes that make their possessor's existence logically impossible). Soft atheists therefore usually say something to the effect that the currently available evidence offers no compelling reason to believe that any supernatural agency is behind anything we see in the real world. That's what most modern atheists, myself included, believe. It may sound like something Star Trek's logical Mr. Spock might say, but that doesn't make it wrong.
Just to muddy the waters a bit, it may interest readers to know that many atheists don't even like the term "atheist." We'd rather be described by the things we believe in rather than the ones we don't. I would prefer calling myself a humanistic or metaphysical naturalist (I'll save the definition of that for some other occasion) if it didn't draw such blank looks from people. So alas and alack, unless we want to give longwinded explanations every time someone asks what we believe, we're stuck with terms people recognize... as imperfect (and imperfectly understood) as they often are.