The rotator cuff is commonly talked about in regard to injury or strength training.
The cuff is basically four inner layer muscles that rotate and stabilise the upper arm bone in the shoulder socket.
The four muscles consist of the S.I.T.S group. The Supraspinatus, Infraspinatus, Teres minor and Subscapularis. The picture below outlines how these muscles connect to the upper arm bone (head of the humerus) working to rotate and support it. Looking at the fingers in the picture, imagine contracting each finger separately and how doing that would move the bone in different directions.
The most commonly injured or torn rotator cuff muscle is the Supraspinatus situated on the top. This is basically because the supraspinatus muscle helps to lift the arm up - a movement we tend to do a lot of.