From reference.com:
An atheist is one who denies the existence of a deity or of divine beings.
An agnostic is one who believes it impossible to know anything about God or about the creation of the universe and refrains from commitment to any religious doctrine.
No one can prove or disprove God's existence. Either you believe or you don't. For both believers and non-believers, you have to have faith in your own beliefs. An atheist has to believe there is no God, while an agnostic just says that they don't know enough to make a decision and thus are not required to believe either way.