There's a couple definitions available.

Everyday usage of Agnostic — This version basically means one who doesn't know if there's a god or not.

The classic Agnostic — This would be one who believes that the god is unknowable, or cannot be investigated.

In reality, the agnostic who uses the everyday definition is basically an atheist.

Differences exist between an agnostic and an atheist, although they aren't mutually exclusive.