A hip sprain is caused by tearing or stretching the ligaments that surround the hip, joining one bone to another. This is different to a hip flexor strain, which is an injury to the muscles that you use to lift your knees, including the iliopsoas (iliacus and psoas muscles) and the rectus femoris, part of the quadriceps.
While hip strains can occur quite commonly, hip sprains are very rare. Most hip sprains will get better at home if you care for them correctly.