source: http://study.com/academy/lesson/bursa-definition-anatomy.html
Bursae are fluid-filled sacs located around a joint that cushion bones when muscles, tendons, or skin rub against the bone. They are very important in movement of the bones because they help reduce friction. Without them, movement would be difficult and even painful. Now let's try and relate this to something you may know about.
Let's say your friend told you about an incident that happened last week in the factory where he works. One of the machines made a very loud grinding sound and then suddenly stopped working. On inspection, your friend tells you that the washers that lie between the metal parts of one of the machines broke. The grinding was the sound made when metal rubbed on metal. In this example, the washers were crucial to having the machine work. Likewise, the bursae are crucial for having the joint work properly
Anatomy
Wherever there are tendons moving across a bony surface, there is a bursa. There are approximately 150 bursa found in the body. Most are present at birth, but a bursa may form in an area where there is friction. Bursae are filled with a thin layer of thick fluid (like the consistency of a raw egg white) and a lining of synovial fluid, which also works as a 'shock absorber' for the joint and supplies molecules to the cartilage. This is known as 'dissipating force'. Normally, the bursae are thin and provide a very slippery surface so there will be very little friction when there is movement of the joint.
Think of a slide on a playground. If a child has a piece of wax paper and sits on it to slide, they go down very quickly don't they? This would represent the function of a bursa. But if the child has on shorts and tries to slide down - the skin can cause more friction and the child can hardly move down the slide. If no bursae are present at the joint, bending your elbow would be like trying to go down a slide with shorts on.
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