The knee is a complicated little piece of the human body.   It is a joint comprised of bones, ligaments, tendons, and cartilage.  All of these things join together to give the knee function and allows us to bend and walk, as well as just stand up and bear our own weight.  The knee joint lets our leg bend backward and move slightly from side to side.
Knee Cap
The knee cap is the part of the knee that most of us are the most familiar with.  It forms that knobby protrusion mid-way up our leg.  The knee cap is a bone that slides along a groove in the femur and is held in place by tendons on the top and ligaments on the bottom.  Most of us just refer to it as our knee cap but the true name for that bone is the patella.
Four ligaments work together in the knee.  These ligaments connect the lower leg bone (tibia) to the knee.  The ligaments are called the PCL or posterior cruciate ligament and the MCL or medial collateral ligament, as well as the LCL or lateral collateral ligament, and the ACL or anterior cruciate ligament.
Tendons connect the knee cap to the femur which is the bone in the top of the leg.  The hamstring tendons are two groups of tendons that connect the tissue at the back of the knee and thigh.  They connect the muscles to the knee.  This allows us to pick up the bottom half of our leg and bend it back at the knee.
Cartilage in the knee helps reduce friction where the tibia and femur connect to the knee.  It acts as a cushion and barrier against pain and injury.  The cartilage in the knee is formally called the menisci.