Atheism
Deism
Deity
Freethinker
God or gods
Secular Humanism
Theism
An agnostic is someone who believes that the truth about something is unknowable. The term is used most often to refer to the belief in gods (or God). Agnosticism is not the same thing as uncertainty or indecision or even proof. It refers only to whether it is possible to know something.
When it comes to religion, agnostics come in two flavors. Agnostic theists are those who choose to believe in a god "just in case." Agnostic atheists are those who choose not to believe until otherwise persuaded.
More Info:
Agnosticism 101 (About.com)
Are you agnostic? (AlabamaAtheist.com)
Atheism
The word atheism literally means "without gods" or "without God." It depends on who is using the word.
Self-identified atheists include those who reject a belief in gods (or God) as well as those who never had such a belief in the first place ("without [a belief in] gods"). To the theist or believer, an atheist is someone who has not yet discovered God, who has denied God, or who is estranged from God (literally "without God"). The possibility that God does not exist is not even imagined.
More info:
Atheism Awareness (AlabamaAtheist.org)
Guide to Atheism and Agnosticism (About.com)
The Atheism Web (infidels.org)
The Skeptic's Dictionary (skepdic.com)