Theism; Convinced belief in a god, multiple gods, a god like being, or multiple god like beings. Simple. Someone who is looking for religion but is not sure which one is still a theist.

Atheism; Convinced belief (technically, certain lack of belief) in the lack of a god, multiple gods, a god like being, or multiple godlike beings.

• Agnosticism; There are several definitions for this term.
• Unsure whether or not there is a god (common usage).
• Sure there is a god, but accepts that there is a small chance he does not exist.
• Sure there is not a god, but accepts that there is a small chance that he does exist.

For an example to the third, as there are little examples to the second and I am sure you can find one to the first, Dawkins, author of several atheist books, is a self described agnostic, under the third definition.