The knee is a pretty remarkable joint. The photo shows a model looking at the knee from the front. The joint, is a hinge, designed to flex and extend in one plain of motion similar to the way a door hinge opens and closes. It also has some very amazing shock absorbing principles, that allow us to run and jump. The joint runs into real problems when we ask it to do things it isn't designed to do like rotate, or hyper extend beyond it's normal range of motion.
There are extra articular structures (outside of the joint) that keep the knee stable. The muscles of the thigh that cross the joint and several collateral ligaments that do the same. You can see these collateral ligaments on the outside of the model.
Inside the knee, there are two cushions of cartilage, called the menisci. The medial meniscus on the inside and the lateral meniscus on the outside. They are labeled in the photo above. Finally, making a cross in the center of the knee are the two cruciate (cross) ligaments. The anterior (ACL) and caudal (CCL) cruciate ligaments. These guys keep the knee from sliding forward and backward. In fact the instability caused by injury to the ACL is called anterior drawer sign because when the muscles are relaxed you can actually pull the tibia (shin bone) forward like a drawer in a desk.