Most of the joints in the body are synovial joints. Let's look at the knee joint in more detail as it can be classed as a typical synovial joint.
Inside the knee joint there are two bones joined together: the femur and tibia. The ends of the bones are covered in a very smooth layer of a tough, rubbery substance known as
cartilage. The joint is enclosed in a stiff and elastic capsule made of strong, fibrous tissue. The joint capsule completely encloses the space around the joint surfaces and is
lined by a synovial membrane. The joint capsule contains a thick, slippery liquid called synovial fluid.
Other types of synovial joints are the ball-and-socket joint in the hip and shoulder, the hinge joint and pivot joint of the elbow, the plane joint in the wrist, the condyloid (or ellipsoidal) joint in the fingers, and the saddle joint in the thumb. Also the joint of the jaw (temporomandibular joint) is a synovial joint.
All these joints allow movement and thus require a lubrication for the joints to function properly: synovial fluid.