[PatrickHenry]

But I still don't see how anyone can think they know something that has to be completely divorced from observable experience.

There is a subtle philosophical distinction. The atheist says: "You -- Mr. Theist -- have no evidence, and until you do, there is nothing for me to consider."

The agnostic looks at the same data and says: "True, there's no evidence for theism, but what do I know? It might be true anyway."

So in this context, the agnostic looks at the absence of evidence and still holds open the likelihood that there may be a case to be made. The atheist doesn't exactly say there's no god (some do, but not as I'm defining it), just that there's no reason for him to even consider the possibility until some evidence turns up. As I said, it's a subtle difference.

[BMCDA]

And thus Atheism and Agnosticism do overlap to a great extent.

[VadeRetro]

There is a subtle philosophical distinction.

In my teens, when I'd recently figured out that I'd been an agnostic for years, I thought that an atheist "knew" that there was no God, whereas an agnostic just couldn't tell and didn't see how everybody else was acting so cocksure.