The most frustrating thing about being an atheist...
There are almost as many definitions of atheism as there are atheists, but all atheists share one thing in common: we simply don't believe in God.
Atheists can be roughly divided into two groups1, strong (or positive), and weak (or negative). A weak atheist doesn't believe in God because he (or she) isn't convinced that God exists. A strong atheist doesn't believe in God because he (or she) is convinced that God does not exist. For most theists, the difference may be subtle to non-existent, but trust me, the difference is real, substantial, and can lead to arguments among atheists.
I personally fall into the weak camp. I cannot say with any certainty that I know that God doesn't exist; if God is everything He's advertised to be, there's probably no test that could conclusively demonstrate His existence one way or the other. What I can say with certainty is that I have not yet had any personal experience that convinces me that God is real. My mind can be changed, but it has to be a pretty unambiguous demonstration of godly power; seeing Jesus' face in a tortilla isn't going to do it for me. Similarly, testimony of the faithful alone isn't going to be much use either. No, it'd have to be God appearing as a 100-foot tall colossus striking down all the lawyers2 with lightning bolts to get my attention. Absent such evidence, my default position is going to be that I don't believe.
I try to respect other people's beliefs for what they are (there are exceptions, but they're fairly rare). All I ask is that people respect my non-belief for what it is, rather than the caricature that they think it is.
1. I sometimes refer to a third group called the Almond Nutbars, who feel that religion in any form is a pernicious institution and should be abolished forthwith. They go beyond militant.
2. Except Doug. Doug's cool.