The knee joint is the intersection of your femur (your thigh bone), your tibia (your shin bone) and your patella (your kneecap).
The quadriceps is the big group of muscles that make up your thigh. These muscles run down the length of your femur and taper off as they approach the patella. They taper down into a tough piece of connective tissue called the quadriceps tendon. This tendon attaches to the top of your kneecap. On the other side of the kneecap, this connective tissue (now called the patellar tendon) continues downward and connects the bottom of the kneecap to the top of the tibia.
The kneecap rests in a groove (the trochlea) at the bottom of the femur, where it's able to slide forward and backward. The kneecap is as much a part of the tendon as it is a bone with two tendons attached. Without the patella, your tendon would directly hug the joint, grinding back and forth in the groove of the trochlea and wearing away with use. As it is, the patella holds the tendon slightly away from the joint, protecting it and maximizing the tendon's function, which is an important one: When the quadriceps contracts, the quadriceps tendon contracts, pulling your kneecap back. This pulls the patellar tendon back, which (being attached as it is to the top of your shin) straightens out your leg. Relax the quadriceps, and your leg bends.
There's more to the picture, though. The shape of the femur and tibia have subtle variations in different people, as do the form of the ligaments. In addition to the quadriceps tendon and the patellar tendon above and below the kneecap, there are four tendons that directly attach the femur to either the tibia or the fibula (the smaller lower leg bone that runs beside your shin). These tendons keep your leg perfectly aligned, adjusting every moment in relation to each other and the movement of the joint. If they didn't exist, not only would walking normally be impossible, your lower leg could very well spin like a top below your knee! The ligaments, tendons, bone and cartilage must all be correctly aligned and remain healthy for the knee to work properly.
So, it's the biggest joint in the body, it supports almost our entire body weight and it has an intricate design -- what could possibly go wrong?