The knee is made up of four bones: the femur, or thigh bone; the tibia and fibula, the two parallel bones in the lower leg; and the patella, or knee cap, which floats over the front of the joint. It basically moves like a hinge.
The knee joint is stabilized by ligaments, bands of tough connective tissue.
Between the tibia and the femur is the meniscus, a cartilage pad that helps the knee absorb shock. The joint is surrounded by a bursa, a sack of lubricating fluid.
While the knee is thought of as one joint, it's really a combination of three joints, or compartments. Two of the compartments are where the rounded ends of the femur make contact with the top of the tibia. The compartment on the inside of the knee is the medial compartment and the outside one is the lateral compartment. The third compartment is the patellofemoral compartment, where the patella contacts the femur.