The collateral ligaments are the 'guy ropes' on the sides of the knee.

The one on the inner aspect is the 'medial collateral ligament' and the one on the outer aspect is the 'lateral collateral ligament'.
What do the collateral ligaments do?
The collateral ligaments restrain the joint from forces that would open it up on the side, ie varus or valgus forces.
-
-
FORUM DISCUSSIONS
CLINICAL PUBLICATIONS
Medial collateral ligament injuries of the knee: current treatment concepts. Chen L, Kim PD, Ahmad CS and N. Levine WN. Curr Rev Musculoskelet Med. 2008 Jun; 1(2): 108–113.