Cleavage is the tendency of a minerals to break along flat surfaces. It means that the make up of the mineral is uneven, dense on one side and not dense in the other, causing the mineral to break along flat surfaces. The tendency of a mineral to break irregurlary is fracture.
Cleavage is the breaking of a mineral along flat surfaces. Fracture, on the hand is just the chipping or "fracture" of a mineral. Cleavage is the tendency to break along flat surfaces, and fracture is the tendency of a mineral to break into irregular pieces.