Two ligaments, one on either side of the knee, prevent the knee from moving from side to side too much:
Medial collateral ligament, which is on the inside of the leg
Lateral collateral ligament, which is on the outside of the leg
Two ligaments inside the joint (cruciate ligaments) prevent the knee from moving forward or backward too much:
Anterior cruciate ligament (ACL)
Posterior cruciate ligament (PCL)
The ACL crosses in front of the PCL, forming an X.
The menisci are pads of cartilage that act as cushions between the thighbone (femur) and larger lower leg bone (tibia), which form part of the knee joint.