The iliotibial band (IT band) is a long connective tissue structure that runs from the hip to the knee. It functions as an accessory hip flexor. It also assists the glutes in preventing the knee from crossing mid-line during heel strike and mid stance of gait.
Excessive tightness in this structure is commonly expressed as pain or tightness at the outer knee or outer-mid thigh ("IT Band Syndrome). However, the IT band is simply a connective tissue structure. The muscle that attaches to it is known as the tensor fascia latae (TFL) and is often overlooked in treatment of these conditions.
If you or someone you know is experiencing IT Band symptoms, it's important to be evaluated by someone who knows how to evaluate function as symptoms here are usually secondary to imbalances elsewhere. This video is a review of how to effectively stretch the IT band/TFL complex.