I agree with Daniel insofar as I think skepticism is "a way of looking at the world."
Agnosticism is best understood by simply looking at its Greek roots, I think:
<i>a</i>- prefix that literally means "no" or "not" or "without."
<i>gnosis</i> means "knowledge."
So, an agnostic by that definition, is someone who is "without knowledge" or someone who thinks there is "no knowledge" that can be had with respect to a particular topic.
Same with athiesm
In sum, I think skepticism is a cognitive framework, a way of thinking, and agnosticism is a result, like Daniel said. An person who's reached agnosticism via his/her skeptical cognitive framework would say something, "I don't know for sure if there is or is not a God, but I do know there's not enough evidence to give me a basis for that knowledge." It leaves the question of God's existence open to new evidence.
In practical usage, I tend to think that those who self-identify as atheists tend to be more forth-coming, that is, "There is not a god." Not to imply that they wouldn't be open to new evidence, but only that they've made a more active decision against God based on the evidence they've seen (or not seen).
In short, I think the two statements sum the difference: "I don't have enough knowledge to know if there is a God." And, "There is no God."
Most agnostics that I've known personally are technically agnostics (with respect to definitions) but functional atheists, as Daniel described himself above.