Circles not shown to scale.
Being sure there is no God(s) is a proper subset of atheism.
One cannot hate God(s) and also be an atheist.
There are athists that neither hate religion nor are sure that there is/are no God(s).

“Sure” in definition C refers to what is sometimes called “strong atheism” which means that one doesn’t merely lack a belief in God(s) but also claims a belief that there is/are actually no God(s). Some may even claim knowledge that there is/are no God(s). This last position is sometimes referred to as “gnostic atheism”.

“Not convinced” in definition D refers to someone not yet having seen adequate evidence for God or gods and without this evidence one withholds belief. A common metaphor of this position would be the celestial teapot orbiting between the Earth and Mars. It is possible there could be such a teapot, but without evidence of one being there, most people would withhold a belief.

Someone who makes the claim that there is not a God would also not be convinced that there is a God – thus be a member of the larger circle.

Edit: 12/14/2009.

Since there are strong agnostics that object to my initial venn diagram, I’ve added another circle that might be more to their liking.

Here, agnosticism would be circle E but without circle C. Agnosticism defined as considering the existence of deities as being unknowable. Thus, some agnostics would say they are not convinced (D but not C) since they can’t know one way or the other. Others may say that it is meaningless to even say they are not convinced since it is a subject in which one could not ever be convinced one way or the other (E but not D). Hopefully that covers everybody.

All I am doing is advocating a “big-tent” definition of atheism. It need not include all of agnosticism.

Others consider (a)gnosticism (about knowledge) and (a)theism (about belief) as being orthogonal to each other. With this orthogonal definition, one could have both (a)gnostic and (a)theistic attributes at the same time.

Edit: 01/08/2010

From discussions at  friendlyAtheist and theBelievingAgnostic it is possibnle to be both an agnostic and a believer. There would be a subset of  “E but not D”  for believing agnostics.