Armor Class (usually abbreviated AC) is a demented RPG measure of how tough you are to hit. Not only is it a half-baked idea, but it's rather perverse -- low is good, high is bad. Armor Class is the "ac" in thac0 (to hit armor class zero).

A naked human is usually said to be AC 10, with clothing you get AC 9. Leather armor gives around AC 7, plate mail 3, and dragon scale mail easily gives -4 or better, depending on the age of the dragon it was taken from. The armor class system is also used in NetHack.

Shadowrun and the other games like it have a much better means of describing how well-protected a being (or structure) is from attacks and incidental damage; in fact, I once tried to transplant the entire Shadowrun combat system into one of my AD&D campaigns (there's a lot to it, I'll just say that). This hybrid system is also an integral part of the MUD I've been coding for the last five years (it WILL not die!).