The knee is the joint that links 4 of the upper and lower bones of the leg, which are called the femur, tibia, fibula and the patella. The knee joint is the largest joint in the body and is an extensive network of ligaments and muscles. Basically, it is a hinge which allows movement of the lower leg in only one direction. The lower leg (tibia+fibula+foot) can swing back from the knee, and that is all.
The knee is held together by ligaments which connect the bones and produces fluid which lubricates the movement at the joint. The meniscus is a cartilage that lies on the inner and outer edges of the upper surface of the tibia. These are the shock absorbers of the knee and supply proper weight distribution.