Clutch Terminology

I stumbled upon a Web site selling aftermarket clutches. I was wondering if someone could clarify some of the terminology involved with clutches. In particular I was wondering what the puck number means and how these affect the clutch. Also what does it mean to be a single plate active clutch vs. twin or multi-plate active clutch.

Look at a picture of a 4-puck or a 6-puck clutch and you'll know what puck means, trust me. Can't help you on the multi-plate thing, dunno what it is.