Here are some anatomical illustrations borrowed from ortho.hyperguides.com:
Sometimes when the rotator cuff tendons are pinched by the underside of the acromion (part of the scapula), rotator cuff tendonitis can occur. This condition is also referred to as impingement syndrome, as the tendons are impinged between the bones.
Frozen shoulder (adhesive capsulitis) is a condition where the ball gets stuck to the joint, thereby making the patient rely more on the other parts of the shoulder joint for any motions, like the shoulder blade moving against the chest. This condition is usually treated by aggressive therapy and takes many months to get better.