This is how I define atheism and agnosticism.
Atheist: lack of belief in god(s).
Agnostic: lack of knowledge of god(s).
So by that definition I'd be both atheist and agnostic. One could also be theist agnostic (believe, but have no "knowledge"); or plain ol' theist (believe in God and believe they have knowledge of God too.)
I guess the stronger atheist position can be taken as "belief in no god(s)" and a stronger agnostic position can be "belief that knowledge of god(s) is impossible to obtain". I don't subscribe to either of these views, however.