The knee joint is actually two separate joints, the weight-bearing tibiofemoral joint between the tibia (shin bone) and the femur (thigh bone), and the patellofemoral joint where the patella (knee cap) slides up and down a groove in the femur as we bend and straighten our knee. The purpose of the patellofemoral joint is to give our quadriceps more leverage when straightening the knee making them more efficient. In fact, muscles that move the knee are some of the strongest in our body.
Whilst in reality the anatomy is a little more complicated, functionally the knee can be considered a simple hinge joint, i.e. it bends and straightens, but has a little bit of give into rotation, and as such the tissues and structures around the joint including ligaments, muscles, and cartilage are designed to handle stress when it is applied in this way.