There is a diverse range of opinions on what exactly the word "Atheism" and being an "Atheist" means.
• My posts on this issue: Part 1 | Part 2 | Part 3.
• Reddit: r/Atheism FAQ
• Richard Dawkins gives a Spectrum of theistic probability.
• Richard Carrier asks, Atheist or Agnostic?
• Wikipedia's Atheism Category and Portal
• Atheism 101 by Austin Cline on atheism.about.com
• Atheism 101 Series by Dangerous Talks.
More on Atheism