My understanding, from discussions with people who teach philosophy, is close to that. Atheists actually come in two flavors. The more extreme form (and rather rare, but usually quite vocal and obnoxious) flatly asserts that no gods exist. This is probably untenable, as it can't be literally true without proving the negative. Some may have arguments they think do the job. I haven't studied this enough, but I've got doubts.
The more common form of atheist says that he doesn't believe because there's no evidence for such a belief. In theory, this type of atheist has an open mind, but of course, opinions vary as to what constitutes acceptable evidence.
The agnostic, as you say, takes a middle ground between theism and atheism, asserting that he has no way to figure out what to believe, so he has no belief. As I see it, this position isn't based on evidence or lack of evidence. Rather, it's based on weak epistemology.
The theist believes on faith, which is belief that is not based on evidence or rational proof. (Although lots of theists assert that they've got convincing evidence, or something that they consider proof.)
How about this: Extreme atheism is a faith; uncommitted atheism is a philosophy, but not a faith.