What is the difference between an atheist and an agnostic?

Atheism and agnosticism are umbrella terms which cover a wide variety of different beliefs and knowledge claims about the existence of God. Atheism simply excludes the belief that God exists, or the knowlege that God exists. A positive atheist would claim to know that God does not exist. In contrast, an agnostic rejects all knowledge claims about God's existence. An agnostic might believe that it is more or less likely that God exists, but they would never claim to know either way.