'What is an atheist?'. An atheist would define the word by breaking it down into its two parts: A = without, theist = one who believes in a god. An atheist will tell you that they are simply people without belief (in god's).
I find this inaccurate because it doesn't take into consideration the various forms that atheism comes in. I prefer a more articulate definition. To simply state that atheism is purely a non-belief (in deity) is a very general definition. Historically you could believe in a god but if you didn't believe the way the local orthodoxy saw it, they would call you an atheist.
I find a three-fold definition of atheism is much more specific:
The first is 'Virtual Atheism' and those adherents in it describe the divine in such indefinable terms as 'the collective consciousness', 'the unknowable', 'personified reality' or as in the Star Wars series, 'the force.' Many naturalists have adopted this view and it has been increasing in popularity.
The second is "Practical Atheism" which is probably the closest to Agnosticism where it's adherents aren't confirmed atheists, but are simply indifferent to god. They may even say 'there may be a god somewhere but he/she isn't interested in me. Some of this category have no idea of what they believe they don't believe themselves.
The third is "Dogmatic Atheism" and the most indefensible and hard core of the positions. Those atheists in this group will tell you that 'god does not exist!' Many of them have determined that all religion is a hoax. An example of this type of atheism is Communism with which the phrase 'religion is the opiate of the people' originated. These groups will fight against all religion and in particular, Christianity. There has been a revival of this type of atheism in America in recent years spearheaded by such groups as the ACLU.