Atheism means non-belief in a god or gods. Yes, there is a subset of atheists that will claim that they know there is no god, but its the simple absence of belief that is the defining thing.

Think of it like this... Theism means: belief in a god or gods. The prefix "A" means "without". A-theism therefore means to be "without belief in a god or gods". If you don't live your life under the assumption that god exists, you are an atheist.

I see agnosticism as a wishy washy term that's so broad in its definition its hardly worth mentioning. How many people say they are agnostic about Zeus? No one right? So on what basis do they dismiss Zeus but remain agnostic about some other conception of god? There is no end to the absurd claims one can make that are unfalsifiable. Other things you can substitue "Zeus" for: fairies, vampires, Thor, invisible pink unicorns, the Flying Spaghetti Monster... you get the point.

Agnostics, I think, want to avoid calling themselves atheist mostly because of the misconception ytfan17 is dispelling and because of the social stigma associated with the term.