Agnostic means 'without knowledge', and atheist means 'without god'. An atheistic stance means you don't believe in a god/gods, while an agnostic stance means you don't believe you can prove god/gods exist or don't exist. So you can believe but not be able to prove that belief (agnostic theist), or disbelieve and not be able to prove it (agnostic atheist), and either way you'll be agnostic.

One is related to knowledge/proof, the other is related to belief.