Most atheists tend to define atheism as a lack of belief in gods. This would include all people who are absolutely sure gods do not exist to people who don't know if gods exist or not. Key word is "belief" as opposed to "knowledge.
Agnosticism deals with knowledge, not belief. People can be agnostic or gnostic in reference to gods or any other thing but its usually in reference to gods.
So people can be any combination of agnostic/gnostic atheist/theist. That pretty much covers all the ground for god beliefs.