Atheist
There are two in-use definitions of the word atheist:

1.) A person who lacks belief in a god or gods.  People who use this definition categorize atheists as either negative (or implicit or weak) atheists or positive (or explicit or strong) atheists.  Negative atheists, while they don't believe in a god, do not positively assert that no gods exist.  Positive atheists, however, do.

2.) A person who believes that no god or gods exist.

Those who consider themselves atheists (who are usually positive atheists) tend to define 'atheist' using the former definition, and those who believe in a god or gods tend to define 'atheist' using the latter.  In both cases, this seems to be a demagogic practice intended to classify either as many or as few people as atheists as possible.  Negative atheists are usually referred to as agnostics.

While neither definition of atheism entails any personality traits of atheists or of those who do believe in a god or gods (sometimes referred to as 'theists'), both atheists and theists tend to believe that certain traits apply to all atheists/theists.  Common myths about atheists include: all atheists are arrogant, immoral, rebellious, and/or intolerant; all atheists really believe in (the Christian) God and are simply in denial; all atheists are depressed and believe life is meaningless.  Common myths about theists include: all theists are naive and/or intolerant; all theists don't really believe in their god(s) and just pretend to because they're afraid that if people realised that no god exists, there will be complete chaos; all theists are weak and look to their god(s) for comfort.  It hardly needs to be said that while these myths apply to a select few individuals in each group, the claim that they apply to every atheist or every theist is false.

Negative atheists justify their lack of belief in god(s) by pointing out that no evidence exists for the existence of god(s), and justify their lack of disbelief by pointing out that no evidence exists for the nonexistence thereof.  Positive atheists often assert that while no evidence for god(s) exists, it should be by default assumed that they do not, until evidence is found.  Some also try to use logic to prove that God does not exist, but this is often attached by theists and weak atheists as being mere wordplay.  Some theists and weak atheists also assert that logic is superficial and meaningless (which is an easily refutable claim), and that only one's inner feelings can lead them to truth.  Some positive atheists (and deists), however, are led to their stance through their feelings.  Some feel that because the world is filled with evil and malice, and all-loving, benevolent god cannot exist.  Those who are led to their stance through this feeling, however, cannot justify their disbelief in a malevolent god in the same way.  Some reject the idea of the Christian God because the Bible states that non-Christians are condemned to Hell, and reject the idea as being too disturbing.  While the same argument can also be made against the gods of various other religions, including Islam and Zoroastrianism, it cannot be made against every god (in Judaism, for example, Hell does not exist).
"Do you believe in God?"
"No, I don't."
"Then you're an atheist?"
"I suppose.  I don't believe God doesn't exist, so I could probably be called an agnostic instead.  Some atheists may consider me a 'negative atheist', though."
"Don't you think atheists... er, would I call them 'positive atheists'?  Don't you think they're intolerant?"
"Yes, you'd call them 'positive atheists', and no, I wouldn't say they're all intolerant.  While some may come off as arrogant, they're just unable to understand how you can believe in God; you probably come off as arrogant to them, also, because you're unable to understand how they can disbelieve in God.  People on both sides really need to make an effort to be more tolerant, and to discuss their stance on religion without using childish, immature insults."