A bursa is a membrane near a joint that acts as a cushion between the muscle and bone. The bursa reduces friction caused by movement and makes the joint more flexible.
Bursitis is inflammation of a bursa. It is a common condition that often happens when a joint is overused, such as when throwing a baseball or painting a wall. More rarely, bursitis can be caused by gout or an infection. Bursitis is more common in people who are overweight, elderly or diabetic, although it also develops in younger, healthy people without a clear reason.