Ankle sprain is the most common injury of the foot and ankle. Sprains most commonly occur with the ankle turning inward, which is called supination or inversion. With this type of injury the outside (lateral) ankle ligaments, which stabilize the small outside lower leg bone (fibula) to the ankle bone (talus) or to your heel bone (calcaneus).

The ligament most commonly torn by an ankle sprain is called the anterior talo-fibular ligament, or ATFL. Severe sprains can also involve the ligament on the inside of the ankle, the deltoid ligament. "High ankle sprains" are the most severe types of sprains involving the ligament that connects the tibia and fibula together (the syndesmosis).

Ankle ligament injuries can range from a simple stretch of the ligament, a partial tear, or a complete tear.

Further Reading:

American Academy of Orthopaedic Surgeons: Ankle Sprains