The rotator cuff is made of the four muscles and tendons of the shoulder (glenohumeral) joint: supraspinatus, infraspinatus, teres minor, and the subscapularis. Damage to any of the muscles or tendons, or inflammation, within this complex structure are classified as a rotator cuff injury. The supraspinatus and infraspinatus are the most commonly injured muscles.

Further Reading:

American Academy of Orthopaedic Surgeons: Rotator Cuff Tears and Treatment Options

BioMet: Arthroscopic Surgery / Rotator Cuff Tear