A bursa is a sac of fluid that sits between muscles and bones or between two different muscles. The purpose of a bursa is to stop structures from rubbing against each other and causing damage. Bursas are especially found around joints where many structures interact and need to be protected so that they do not rub against each other. Instead, these structures rub against the bursa that is made to withstand this rubbing. This picture illustrates the bursa found in and around the knee joint.

Bursitis means that the bursa is inflamed. An inflamed bursa puts pressure on muscles and joints in the area and will cause pain with movement. This can lead to the inability to perform certain movements and functions.