The rotator cuff is a group of muscles that attach via tendons to the head of the upper arm bone (the humerus). The function of the cuff is to center the head of the humerus in the socket and move the arm. These tendons are in a constant state of rebuilding and breaking down.
When the rate of breaking down exceeds the ability of the tendon to rebuild, micro tears occur causing pain and inflammation. This inflammation is known as tendonitis. In the extreme case, this inflammation can result in the failure of the tendon known as a rotator cuff tear. Repetitive overhead activity or heavy lifting can irritate or weaken the tendon.
Sometimes this can also lead to a gradual tear in the rotator cuff tendon making it difficult to raise or rotate your arm. Acute tears can also happen with a sudden force that overwhelms the tendon such as a shoulder dislocation. This results in an immediate inability to raise or rotate the arm.