Your shoulder is a ball-and-socket joint where a ball (the head of the humerus, or your upper arm bone) fits into the socket (glenoid) of your shoulder blade (scapula). The ball at the top of your arm fits into a shallow socket in your shoulder blade.
The rotator cuff, made of four muscles, helps hold the arm in place and allows you to lift and move it. The tendons of the rotator cuff cover the top of the arm bone and help with arm movement.
There’s also a small fluid-filled sac called the bursa between the rotator cuff and the top of your shoulder. It helps the tendons move smoothly, but if the tendons are damaged, the bursa can get irritated and cause pain.