The exact meaning of atheist is often up for debate.

There is one group who defines it as:

One who is without religion. One who does not believe in any god or gods.

Others, however, see it as meaning actively denying all religion, as saying "there are no gods". They see agnostic as being more appropriate for the first definition.

So before jumping to any conclusions about what is meant when someone uses the word "atheist", try and look at context, or even question the person on what exactly they mean. Otherwise you'll argue about something that may not even be in disagreement.

I personally believe in the first, that it means lack of belief, and that an agnostic usually someone who's more uncertain of what they believe.