The achilles tendon is the large tendon at the back of the ankle. It connects the big calf muscles at the back of the lower leg to the foot and inserts at the back of the heel or calcaneus bone. It provides the power in the push off phase of walking and running where huge forces are transmitted through the achilles tendon. Achilles tendonitis is usually an overuse injury caused by doing too much too soon. Strictly speaking the term tendonitis suggests an inflammatory condition of the tendon but in reality few injuries are actually down to pure inflammation. The main finding, particularly in older athletes or persons is usually degeneration of the tendon. The term Achilles tendinopathy is probably a better term to describe the range of conditions that can cause Achilles tendon pain.
Although overuse is the primary cause, there are a number of factors which can increase your risk of sustaining the injury including poor footwear, soft training surfaces, tight muscles and foot biomechanics and running uphill.