Among those who have become athiests (or for those who follow atheism), there two types of atheists, namely:
Strong atheist - One who denies the existence of God (or any gods).
Weak atheist - One who does not accept the existence of God (or any gods).
The difference lies in saying "There is no God" and "I don't believe in God", which are two separate concepts.
Note that when using this nomenclature, one must be an atheist or a theist, there is no middle ground.* Some people label themselves as an agnostic to avoid the label of atheist. But they are separate and exclusive. An agnostic is one who believes the nature of God is unknowable.
*Polytheists and Pantheists are possible exceptions.