Do you believe in God, or do you believe that there is no God*? Just how sure are you? Depending on your answer, you may be an atheist or a theist along the spectrum of surety with agnosticism on one end and gnosticism on the other.
(*Now, someone explain to me whether Zen Buddhists believe in God.)
Confused? You're not alone, but thanks to Pablo Stanley (previously on Neatorama), we now have a handy dandy guide to know what the hell you are:
God - some kind of power, energy or being that works outside of perceivable, measurable existence.
Believe - accepting something without proof.
Exists - something that is (man thats a tricky definition)