A fixed rotor is a single piece unit hard mounted to the wheel making it completely rigid. The floating rotor is a 2 piece unit. The rotor is attached to its hub via rivets and wave washers that allow it to be semi-rigid or floating. The benefit is that when the brake is released this small amount of float will cause the caliper to completely release eliminating the pads from dragging which in turn can cause warpage to the rotor.
