It has to do with the difference between what you believe and what you think you know. For any particular god that you can imagine, a "theist" is one who has a belief in that god. In contrast, an "atheist" is one who does not have a belief in the god. A "gnostic" is one who knows about the existence of god and an "agnostic" is one who thinks that god is unknowable.
Notice that the terms "atheist" and "agnostic", by these definitions, are not mutually exclusive. You could be an agnostic atheist, meaning you don't think that the existence of gods is knowable, but you don't believe in one without further proof. Many people assume that atheists believe that gods can be proved not to exist, but this isn't strictly true and there is no proper word to describe this. You could call such a person an "untheist", perhaps. Or, you could just call such a person a "gnostic atheist", one who doesn't believe in a god and thinks that his non-belief can be proved. Note that there is a difference between an absence of belief in a god and a belief in the absence of god.
So there are four possible ways one could be.
1. Agnostic-Theist: believes god exists, but the existence of a god is unknowable
2. Gnostic-Theist: believes in a god for which he claims knowledge
3. Agnostic-Atheist: does not believe god exists, but it can't be proved
4. Gnostic-Atheist: believes it can be proved that god does not exist
Case 3 is sometimes referred to as "weak atheism" and case 4 is sometimes referred to as "strong atheism". Only strong atheism positively asserts that there are no gods.
Finally, it should be pointed out that when a person is asked about their beliefs and replies that they are agnostic, they are avoiding the question and answering a different one. Someone who can't positively say he/she believes in a god is an atheist.
Posted in: About atheism