Theist = Believes in a god.
Atheist = Doesn't believe in a god.
Agnostic = Doesn't know whether or not god exists.
Gnostic = Does know whether or not god exists.
Agnosticism/Gnosticism and Atheism/Theism are NOT mutually exclusive. Everyone takes one of the two positions regardless of what else they say.
I would just add to the above that the Gnostic can not REALLY know one way or the other. They can only have "faith." (If a Gnostic can prove conclusively that there is a God(s)/Goddess(es), then that will be a historical first for humanity.)
So, I'm an Atheist AND an Agnostic. Fun!
More of my guy--
Penn Says: Agnostic vs. Atheist