The iliotibial (IT) band is a tough group of fibers that run along the outside of the thigh. The gluteal muscles and the tensor fascia lata muscle attach to the top, and the lower part attaches to the tibia, just below the knee. It functions primarily as a stabilizer during running and can become irritated from overuse. Runners will usually describe pain on the outside part of the knee or lower thigh, often worsened by going up or down stairs, or getting out of a car.
Treatment consists of relative rest, ice massage, addressing any biomechanical or training errors, and gentle stretching . When doing the IT band stretch the "pulling" sensation is usually felt along the mid to upper thigh. Don't worry if this doesn't seem to exactly hit the site where the pain occurs. The IT band is a long structure and the goal is to get it to loosen and lengthen. Along the same lines, gentle stretching of the gluteal muscles will also help. (Please see the stretching section -- especially the additional IT band stretch.)
Prevention of the IT band syndrome is achieved by running on a level surface or alternating directions on the road, a balanced approach to training which allows for rest and recovery, and preventive stretching. In some strengthening the external hip rotators will help. This is achieved by doing sets of one leg squats in front of a mirror. Watch pelvic alignment to ensure that one side does not drop. Finally, orthotics can be quite useful if you have a tendency to develop IT band inflammation.