A bursa (plural: bursae) is a small fluid-filled sac lined by a synovial membrane with an inner capillary layer of synovial fluid. These provide a cushion between bones and tendons and/or muscles around a joint. This helps to reduce friction between the bones and allows free movement. Bursae may or may not communicate with the joint space.
Examples
According to location
According to etiology
deep bursae: present from birth
superficial bursae: form shortly after birth
adventitial bursae: acquired due to friction