As I understand it- motion in a circular path requires a constant acceleration towards the centre of the circle. A constant acceleration towards the centre requires a constant force towards the centre.
That force is centripetal force and is applied by the piece of string on the stone or whatever. Centrifugal force is the 'reaction' to centripetal force (As in "every action has an equal and opposite reaction")and is the force felt by your hand when you are spinning the stone round. Now I can't remember what the question was!!!