I was asked what an agnostic was by a friend on Friday, and since the atmosphere was not conducive towards long philosophical discussions, had to come up with something of a one liner:
Atheists are people who believe that the lack of evidence for the existence of God is evidence enough of its non existence. Agnostics see the logical flaw in that approach.

Probably not the most profound and comprehensive definition, but a good one to have lying around to use in a hurry.