Rumblood doesn't understand the difference when he asks "Is there a Canadian quarter in my pocket?". I do not know. I can guess.
Framed as "Do you know that I have a Canadian quarter in my pocket?" the answer is no. This makes you an agnostic regarding the contents of his pockets.
If he would've asked "Do you believe I have a Canadian quarter in my pocket?" I would say no, because I have no reason to do so. Same goes for "Do you believe my pocket is empty?". No, no I do not, why would I go around believing that? There is no proof for that claim either. Once again, if he says he has one, I'm willing to accept on the grounds of "Who cares?", but at the same time I'm not walking around believing what he has in his pockets due to the same principle.
Take that into the context of deities, you will see that I am an atheist by the mere virtue of not believing in a god. Since I also do not know whether a god exists or not, I am also agnostic.
The point is, I know what I believe, and I know what I know. I neither believe, nor do I know.
EDIT:
To make it even more clear, if someone asks "Do you believe in gods?" answering "I do not know" is strange, since it does not answer the question of whether he believes or not. The answer is usually "I do not believe in the existance or non-existance of gods". Then if you want you can clarify that you require knowledge before belief in either claim, however due to the fact that he does not believe, makes the person an atheist. An agnostic atheist in this case.
An agnostic theist would answer yes. He does believe. But since he is agnostic, he does not in fact know. A gnostic atheist, would claim to know, and would probably also answer that "I believe that god does not exist." However his answer to the intial question "Do you believe in gods?" would be the same as mine. No.
