The rotator cuff is made up of 4 muscles (the supraspinatus, infraspinatus, teres minor and subscapularis) which act as the dynamic stabilizers of the shouder to control the position of the head of the humerus within the glenoid fossa.
A shoulder impingement occurs when one of the tendons of the rotator cuff muscles gets impinged within the joint. This causes the tendon to get inflamed and damaged.
The rotator cuff can also develop a tendinopathy, which can be caused from either an overuse injury or from an excessive overload due to poor mechanis.
For both conditiond it is important to see a physio therapist to help with the pain and inflammation and most importantly to get a programme to correct the shoulder biomechanics to prevent further injury.