Your Iliotibial Band (IT Band) is a fibrous connective tissue that surrounds the lateral thigh.  When this connective tissue (ligament) extending from the pelvic bone to the shinbone becomes tight, it rubs against the thigh-bone often causing pain between the hip and the knees. 

Regularly stretching the IT Band can greatly reduce pain in the knees and hips.  For all you runners, this is definitely a post you’ll wanna save.