An ankle sprain is a common injury that occurs when ligaments of the ankle are partially stretched or completely torn. With a partial stretch the ankle will continue to be somewhat stable, but when the ligaments are completely torn the ankle will lose stability in that area. This injury can be caused by sudden stretching or twisting to the ankle from a sport or when walking on uneven surfaces. Pain and swelling are the initial symptoms with a sprain possibly followed by a popping sensation, instability, or bruising.

Source: medicinenet.com