Clutch parts-what are they & what do they do?

i eventually want to upgrade from my stock clutch, but i know nothing about clutches. pressure plate, flywheels, stage I,II, III, what does this mean? simply put, how does a clutch work? what will i need when i get around to replacing my stock clutch?

http://auto.howstuffworks.com/clutch.htm

Stage I II III are different between the companies. Check out centerforce or ACT websites.