That's quite an interesting thing to ask. We've recently talked about this topic in our religion classes and as far as I got it, there are four different kinds of atheism.
1) Practical Atheism: The exclusion of God/religion from one's life (but not explicitly stating, that one doesn't believe in God). It is possible that one theoretically is believing.
2) Methodical Atheism: When searching for the causes of natural processes, one has to search for the direct natural causes (the second causes) given by the autonomy of the corresponding natural fields. God can be seen as a given first cause, the reason for the whole reality. (E.g. when you believe in the big bang and see it as God's will)
3) Atheism as form of correction of a wrong interpreted theism, possibly a religious person who believes in some form of higher force etc.
4) Actual Atheism: The rejection of the idea that God exists. (e.g. Friedrich Nietzsche)
For me atheism is: Denial of God.
I'm sorry that this is a lot more than three words, but I wanted to share the information.
Atheism is taught, as religion is, and found when God is abandoned. The atheists I know either gave up on God because they think so many horrible things have happened to them that God must not exist, or they grew up in atheist families and it's simply what they were taught to believe.
Not really, Atheism is the default. Nobody is born believing in God. That stuff is taught.
Anyway, that's rather interesting, The only atheists I know just weren't convinced by religious ideals/ideals/ lack of proof etc. Doesn't sound like these atheists you met have really thought their position through if that's all they are going on.