Basic explanation:
Vr - rotation speed; the speed at which you should rotate the aircrat (it should be at least 1,1 * Vmu if I remember correctly)
Vmu - minimum unstick speed; minimum speed at which you can lift the nose wheel off the ground (but the aircraft is still unable to fly though)
Vlof - lif off speed; speed at which main gear leaves the ground after rotation
Vmu < Vr < Vlof