Not an expert here by any means but the way of using atheism and agnosticism that I've been finding most useful is as follows:
Theism is asserting that one has belief in a deity. Atheism is not asserting this belief. Gnosticism is asserting knowledge and agnosticism is not asserting knowledge. With these definitions agnosticism and atheism are not exclusive, in fact most atheists are agnostic atheists as they will tell you that they don't believe but not that they know there isn't a deity. Most self described agnostics agree that they are agnostic atheists according to these definitions. I've tended to find, among non-philosophers anyway, that the ratio of gnostic theists to agnostic theists is much greater than the ratio of gnostic atheists to agnostic atheists.