The knee is a large and complex joint made of bones, muscles, ligaments and cartilage. The bones could not move without the action of the large muscles that attach to the front and back of the knee. Those movements would be all over the place without the ligaments limiting motion and acting as stabilisers. This is why, in normal circumstances, you can't bend your knee sideways (and it hurts when it happens by accident!)
Cartilage is also important for knee stability and acts as a shock absorber.
All these structures are susceptible to injury, either together or separately. The most common injuries to the knee are sustained in sports or with strong twisting movement. The knee, like any other joint, is also susceptible to wear and tear.
A lot of knee injuries can be avoided by strengthening the large leg muscles.
Keeping those muscles in balance with each other will help stabilise the joint further. See overleaf for ideas of exercises to protect your knees or to help with recovery.