Iscial tuberosities. The "sit bones" are part of the pelvis called the ischial tuberosities. The hamstring muscles also attach to these bones and there is a "bursa" which is a sac of fluid which protects the tendon of the muscles from rubbing against the bone. Sometimes these bursa get irritated and can cause pain where you sit. Also this is a common place for hamstring tears. Many treatments available to help.