Actually, there are two types of each:
- Atheism
- Implicit atheism states no belief in God but no disbelief either. Synonymous to nontheist.
- Explicit atheism is certainty that there is no God. Achieved not through lack of evidence for God's existence but through logical (or sometimes empirical) contradictions in God's existence.
- Agnosticism
- Atheistic agnosticism claims that it's unknowable whether or not God exists.
- Theistic agnosticism claims that God does exist but that his nature is unknowable.
Therefore, there's more than one differences between atheism and agnosticism. It depends on which type of atheism and agnosticism you're referring to.