Ankle sprain The Ankle Ligaments Injured by an Ankle Sprain
There are three lateral ankle ligaments in all. The most important are the anterior talo-fibula ligament (ATFL) and the calcaneo-fibula ligament (CFL). If excessive inversion (movement towards to the mid-line of the body) occurs then most commonly the anterior talo-fibula ligament is injured, but in 20% of cases approximately so is the calcaneo fibula ligament. The anatomical diagram below shows these two structures as discrete entities. They are usually actually in continuity with the lateral ankle capsule. 
Interactive Foot and Ankle 2 2000 Primal Pictures Ltd
The Anterior Talo-Fibula Ligament(ATFL)
The Calcaneo-Fibula Ligament(CFL)
(The fibula, the peroneal tendons)
What else can be injured when an ankle sprain occurs?
The same type of forces around the ankle can produce an ankle fracture, crush the lining of the joint resulting in chondral (cartilage) or osteo chondral injury(1).
Bleeding into the joints itself is common after any of these injuries and this can lead to chronic inflammation setting up in the soft tissues within the ankle joint known as synovitis. 
Alternatively the bleeding may eventually lead to scarring which remains within the ankle joint and this scar tissue can interpose itself between the bones of the ankle and produce pain and feelings of instability.
More rarely the peroneal tendons can become dislodged from their normal position behind the fibula, a condition known as peroneal tendon subluxation.
What symptoms might I have with an ankle sprain?
The initial symptoms following an ankle sprain are pain, swelling and subsequently bruising which may affect both sides of the joint. Additionally, it is not unusual to find it painful to put weight through the joint and the joint may also feel immediately unstable. Pain on attempted weight bearing after a presumed ankle sprain is something which requires immediate medical review. Pain to the touch of any of the ankle bones or an inability to put weight through the ankle following ankle sprain necessitates an x-ray examination (the Ottawa ankle rules).
How common is a sprained ankle injury?
It may be as common as to account for 25% of all musculoskeletal injuries. Another way of looking at it is that one injury occurs per day per 10,000 of population.
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