Originally Posted by ChrisSlicks

In addition to the "less rotating mass" it also means less unsprung weight. The 2-piece front rotors are more than 10 pounds lighter per corner on average.

2-piece rotors have an aluminum hat (preferably billet) and a iron ring. They can be bolted together in different ways, either fixed mounts or full floating mounts. The full floating setup allows for a larger temperature differential between the ring and the hat as it allows for more expansion, however it requires more maintenance, it is typically only applied in racing setups. The fixed mount setup can move as well because the 2 metals will expand at different rates, however their movement is limited by design.

2-piece rotors in the rear application may use an iron hat in order to have a more durable e-brake functionality. If you are only using the e-brake as a parking brake an aluminum hat is perfectly acceptable provided an appropriate thickness is used.