As others have said, I don't see why this is so hard either. The only real confusion that makes sense to me is that many people are used to a single definition of the word and can't seem to get that one out of their head. Depending on what dictionaries you use, both the following definitions are technically accurate (as are several more I won't mention):
1) atheists actively disbelieve in God, just as many of us disbelieve that the Society has the "true religion." If asked whether any gods are real, an atheist by this definition must answer, "No. Without question." This seems to be the definition that many people can't let go of. Still, we atheists must remember that it's a valid definition too.
2) atheists are people who do not believe in God, but don't necessarily cross the line into disbelief. You might not believe that vitamin C cures cancer because you haven't seen proof, for instance, but still withold final judgment until proof is offered one way or the other.
In other words, atheists these days see belief in gods as on-off switch. There is no fence, like agnosticism is supposed to be, to sit on. Either you believe or you don't. Disbelief isn't a factor. If you don't believe in any gods--or are simply unconvinced that any gods exist--then we consider you an atheist. That's it, really. The more I talk about it, the more complicated it will seem.
Most atheists don't consider agnosticism to be a fence between theism and atheism. It is something else entirely. An agnostic believes that you can't prove or disprove gods absolutely. But you don't have to have proof to believe or even disbelieve. So you can be an agnostic Christian (believe in God without believing he can be proven or disproven) or an agnostic atheist (not convinced any gods are real, but without believing this can proven either way). This is why defining agnosticism as a middle ground between the two doesn't work very well, at least to me, and it sounds too wishy washy for atheists who have chosen to be bold and "come out" as nonbelievers. So most of us don't like to use it.
Most self proclaimed atheists are actually agnostic atheists these days, also known as weak atheism. These are valid philosophical terms. If you look up "weak atheist" or "agnostic atheism" you will find definitions for them online or in books that discuss it.