Bursitis is the inflammation of a bursa, a kind of fluid-filled sac that is located between the bones, or between bones and the tendons or muscles, thus facilitating mobility by reducing friction; that is, it serves to cushion the movements. 
The cavity of the bursa is covered by the synovial membrane-membrane, which is a continuation of the articular bursa and secretes fluid for lubricating the joint. It is often caused by the overuse of joints, by an increase in the activity level on a joint, e.g., training for a marathon or being overweight. Bursitis can also be caused by rheumatoid arthritis, trauma, gout, or infection. Sometimes the determination of the cause may be difficult
We will look at, in details, the common cause of bursitis;
Gout is a chronic inflammatory joint disease with recurrent bouts of acute pain.  The pathology is associated with the deposition of uric acid salts, which crystallize in the tissues and deliver damage to the joints with gout. The disease is typical for elderly patients, and it is more commonly seen in men. The joints of the fingers, elbows, hands, knees, and feet can all be affected by this disease. Most often, blood uric acid levels rise to abnormal levels when the kidneys are unable to excrete enough uric acid in the urine. This cause is usually related to the person’s genes. Excess uric acid in the blood can lead to the formation and accumulation of uric acid crystals in and around joints, which can also lead to bursitis.