The short version:
Atheists are against religion and they tend to lead arguments against it. They could be described as Anti-religion.
Agnostics could not care less about religion, they just don't believe. They are not against it, but not for it either. The logic with Agnostics often tends to be: there's no proof of god, but there's no proof against one either.
EDIT: There are many different gray areas in-between and around these, and according to OldTrees, a whole other term for people directly opposed to religion. In my personal experience, this usually applies to people who put themselves under the label Atheist, but don't make presumptions.