When talking to people about atheism I often find them confused about what atheism actually is. All the people I know that call themselves atheists do not claim to know for certain that there is no God, but some seem to argue that due to this uncertainty these people are not atheists but rather agnostics. This diagram should help to clear up the confusion:
I originally found this diagram here: http://www.noforbiddenquestions.com/2010/10/defining-agnosticism/
Let me know what you guys think and where you might fall on the diagram!