Bursitis is an inflammation of a bursa, which is a fluid-filled sac located near the bones around the joints and between muscle attachments at a joint. They cushion and lubricate the movement of tendons and muscle over bone. Bursitis is not arthritis; arthritis is a change within the joint and bursae are outside of the joint.
The joint most commonly affected with bursitis is the shoulder. Other joints that may develop bursitis include the elbow, the knee (this is also known as housemaid's knee), the hip, and the base of the big toe (part of what is called a bunion). Bursitis may be acute or chronic. With the chronic form, symptoms of pain and swelling tend to come and go over cycles of weeks or months.