Sandwiched between the rotator cuff muscles and the outer layer of large bulky muscles is a structure known as the subacromial bursa. Bursae are everywhere in the body. They are found wherever two body parts move against one another and there is no joint to reduce the friction. A bursa is simply a sac between two moving surfaces that contains a small amount of lubricating fluid.
Think of a bursa like this: If you press your hands together and slide them against one another, you produce some friction. In fact, when your hands are cold you may rub them together briskly to create heat from the friction. Now imagine that you hold in your hands a small plastic sack that contains a few drops of salad oil. This sack would let your hands glide freely against each other without a great deal of friction.
Inflammation and swelling of the subacromial bursa is known as bursitis and associated with Subacromial Impingement of the Shoulder