The rotator cuff is a group of muscles which help to control the movement of the shoulder's ball and socket joint. If this control is inadequate then the muscles can become stressed, causing pain in the shoulder or upper arm, particuarly when lifting the arm, lying on it or using the sore muscles.
People who work or do sports and/or hobbies that require putting their hands above shoulder height are the most likley candidates for rotator cuff problems. However, a shoulder injury, new or repetitive activity may also cause the condition to develop. Age also has an effect, with rotator cuff problems developing as you get older.