From the FAA definitions:
Vr Speed. Vr is defined as rotation speed and is
applicable to transport category airplanes certified under
SR 422A and later rules and commuter category airplanes.
Vr is determined so that V2 speed is reached before the
aircraft reaches 35 feet above the runway surface. Vr may
not be less than Vmu or 1.05 Vmca.
Vef Speed. Vef is the airspeed at which the critical
engine is assumed to fail. Vef is selected by the aircraft
manufacturer for purposes of certification testing,
primarily to establish the range of speed from which V1
may be selected. Vef may not be less than Vmcg.
Vmu Speed. Vmu is defined as minimum unstick
speed. Vmu is the minimum speed demonstrated for
each combination of weight, thrust, and configuration at
which a safe takeoff has been demonstrated.