What is the difference between atheism and agnosticism?

While the common perception of an agnostic is a “fence sitter”, who neither believes nor disbelieves in the existence of a god, the term really means that a person doesn’t claim to know whether a god exists, or not. One can be both an atheist AND an agnostic. The following site explains that in more detail:

Atheist vs. Agnostic – What’s the Difference?

This diagram may also help:

AtheismAgnosticism

Another view is that given by Richard Dawkins’ Spectrum of theistic probability