The collateral ligaments are the ligaments on either side of the knee joint.
The medial collateral, in contrast, is flattened into a band with two layers - deep and superficial - the deep layer is attached to the medial meniscus, while the superficial layer attaches to the tibia, and these layers contribute to the joint capsule in this area. It contributes largely to stabilising the bent knee against against any force tending to force the joint open on the inner aspect (valgus force).
If one looks at the back of the knee on the lateral side, the anatomy is complex. The lateral collateral, unlike the medial collateral, does not form part of the capsule. This is because the popliteus tendon passes under it and separates it from the capsule. At this point the capsule is open, and this area is weak and prone to injury. This is a real problem as these structures contribute considerably to what is called the 'postero-lateral corner' - a number of tricky structures stabilising the knee on the outer side towards the back and postero-lateral corner deficiency is a common cause of continued instability after AC
The Anatomy of the Medial Part of the Knee. LaPrade RF, Engebretsen AH, Ly TV, Johansen S, Wentorf FA and Engebretsen L. J Bone Joint Surg Am. 2007;89:2000-2010.