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:




two piece:



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).