Q: What is agnosticism?
A: Agnosticism refers to the philosophical position that it is not possible to know whether or not a god or gods exist. This is a separate issue from whether or not the person in question actually has a personal belief in a god or not. Someone can be an agnostic and believe in a god at the same time -- they simply believe that while no one can know for sure, they personally do have a belief. This person would be a theist. On the other hand, someone can be an agnostic and not have a belief in a god -- while no one can know for sure, they personally do not have a belief.
Theists and atheists can be agnostic, or they can reject agnosticism, taking the philosophical position that not only is it possible to know whether not a god or god exists, but that they in fact actually know.
An agnostic who doesn't make a call one way or the other -- after all, why commit if you can't know? -- would, under the very general definition of atheism as "an absence of god-belief", be considered an atheist.
One of the key points is that agnosticism is a separate axis from the presence or absence of actual belief in the individual. It is a statement about whether or not the existence of deities is knowable, rather than about whether the person in question is a believer or not.