maybe they're both not very theistic.. but here's a good link that explains the difference between the two http://www.diffen.com/difference/Agnostic_vs_Atheist
that's basically it. i'm an agnostic because how the fuck could i be so sure that a "god" does not exist? i try to be objective, and the truth is that there's not enough evidence to point me one way or another. as an agnostic, i see atheists and religious people as the same; they are both people who draw an objective answer without sufficient evidence... so i don't agree with lumping agnostic with atheist.