Whats the difference between 1 piece rotor and a 2 piece rotor?

I believe the 2-piece ones are made up of the "hat" (or bells?) and the "rotor" itself mounted together. An example would be some Wilwood/Stoptech ones. One piece is well, one piece like most aftermarket rotors (EBC, etc.)

one piece:

http://img.photobucket.com/albums/v682/Sullust/bb001.jpg


two piece:

http://www.tcklineracing.com/PF2pcRotor.jpg

note: not all 2 piece rotors look the same, but pretty much similar. also, 2 piece rotors are much more expensive since they are usually larger than stock/aftermarket one piece rotors, and they're usually only made for BBK's (big brake kits).