Atheist: One who disbelieves or denies the existence of God or gods.
Agnostic: One who believes that it is impossible to know whether there is a God, one who is skeptical about the existence of God but does not profess true atheism.
I’m not advocating fanatical disbelief. Certainly, were I magically summoned in front of a god, I wouldn’t close my eyes, cover my ears, and yell, “I can’t see you!” in manner of a two year old. In fact, were he/she/it to do something conclusive like cure world hunger or turn my house into a pillar of salt, I might actually believe I wasn’t experiencing a caffeine-withdrawal induced hallucination* and was instead viewing the powers of a divine being. See? I’ve left open a door for any god interested in developing in a personal relationship with me (interested gods: please see ad below). However, I’m sure enough that this won’t happen to label myself as an atheist rather than just an agnostic. I’m agnostic about god in the same way as I’m agnostic about unicorns, flying pigs, and faithful men**.
* If you doubt the plausibility of this, then you haven’t experienced the true powers of caffeine, and it would be utterly impossible for me to put the glory of such into words.
**Yes, yes- I was just kidding with that last one.