They are structures made of connective tissue, connecting two bones to each other. These ligaments pack the joint, support joint capsule, ensure endurance and movement of the joint. They help stabilization of the joint by allowing movement to certain directions. 

While the knee joint has an anatomy that makes it impossible to be stable in consideration of the bone structure only, it becomes a stable structure thanks to the ligaments of the knee joint.

We help duties of the knee ligaments by developing our muscles but we cannot perform their duties completely. Because: Muscles are connected to the bone with tendons and as long as our muscles are contracted, they can help solidness of the knee but when we relax, in other words, when our muscles are not contracted, they cannot provide this help and knee slipping occurs easily due to existing ligament deficiency. On the other hand, ligaments are tissues that connect bone to another one and contribute to the solidness (stability) of the knee every time (even if we are relaxed). 
 

There are four basic ligaments in the knee;Anterior Cruciate Ligament, Posterior Cruciate Ligament, Medial Collateral Ligament,

Lateral Collateral Ligament.