The only part of this answer I would disagree with is "when someone calls him/herself an atheist, they usually mean they are a gnostic atheist" -- Most of the people who call themselves atheists that I know (including myself) are agnostic atheists. They don't apply the agnostic descriptor because it's not the answer to the question that they are being asked, which is usually something like "do you believe in god?", not "do you know if there is a god?".