An agnostic is unsure whether God exists and requires more evidence before coming to a decision. In that sense agnostics do not believe in God. [from the Greek Gnosis meaning knowledge]
A negative atheist is someone who does not believe that there is a God because they have never really thought about it.
A positive atheist has thought about it and has come to the conclusion that there is no God to believe in. [from the Greek Theos meaning God]