The iliotibial band is an important, yet easy to aggravate, structure in the leg.  The band itself is a very thick, fibrous tissue that arises from two main muscles in the hip, the gluteus maximus and the tensor fascia lata.  There are two bony areas in what people refer to as 'the hip.'  The pelvis is made up of two bones, a right and a left ilium (read, il-e-um), that come together in the back and attach to the sacrum.  The top of the ilium on the outside is just below the ribs.  Just below the bony part is where these muscles attach.  These muscles work to move the leg backward and outward when the leg is swinging and they stabilize the pelvis on the leg when standing or weight bearing.  Their attachments to the iliotibial band come together around the other bony part, the greater trochanter, which can be felt on the side at about the level of your side pants pockets.  

The main hip flexors are the iliacus and psoas (the 'p' is silent) major.  Picture your pelvis as a bowl.  At the back of the bowl is where the lumbar spine attaches, going upward, sitting on the sacrum (5 fused vertebrae forming an inverted triangle) that is very strongly attached to the two halves of the pelvis.  The iliacus muscle starts along the back part of the ilium and sacrum, travels along the bottom of the pelvis and over the front edge to the inside of the femur (thigh bone).

The piriformis is a small but strong muscle that goes from your upper sacrum and illium outward to your greater trochanter (the bony part on the outside of your hip that you can usually feel).  It is a deep muscle that is underneath the large gluteus maximus (buttocks) muscle that we sit on.  The piriformis muscle crosses over the sciatic notch of the pelvis which is where the sciatic nerve exits to travel down your leg, where it carries all the messages to and from all parts below the waist.