I while back, I wrote this blog post about how I viewed the distinction between agnosticism and atheism, and offering a definition of atheism that I thought stood up to scrutiny in the world of discourse about such things. And despite some argument from some, I still hold that the best definition of atheist is someone who lacks beliefs in any gods.
Then just today I watched this video, whch makes many of the same points, and does so in a very tight little presentation.
It is a new video in a great series by Evid3nc3, all of which I highly recommend to theists everywhere. He does a great job of charting his course from being a Christian with questions and becoming an atheist.
Skepticism, Properly Applied 
It seems to me that the difference between agnostics and atheists is just as often in how they feel about theists as in how they think about God. Those who wish to be less confrontation with theists tend to favor the term agnostic.