This article is from the Atheism FAQ, by mathew meta@pobox.com with numerous contributions by others.
They don't believe in God. That's all there is to it.
Atheists may listen to heavy metal -- backwards, even -- or they may
prefer a Verdi Requiem, even if they know the words. They may wear
Hawaiian shirts, they may dress all in black, they may even wear
orange robes. (Many Buddhists lack a belief in any sort of God.) Some
atheists even carry a copy of the Bible around -- for arguing against,
of course!
Whoever you are, the chances are you have met several atheists without
realizing it. Atheists are usually unexceptional in behaviour and
appearance.
 
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