What is the Achilles Tendon?
The achilles tendon is the band of tissue that runs down the back of your lower leg and connects your calf muscle to your heel bone. The Achilles tendon aids in walking by helping raise your heel off the ground. 

What is an Achilles Tendon Rupture?
A rupture of the Achilles tendon is the complete or partial tear of the tendon that happens when it is stretched beyond its ability. This can be caused by excessive or forceful jumping, pivoting on the ankle, or sudden accelerations while running. It can also occur due to falling or tripping. While these injuries most commonly occur during sport-related events, certain illnesses or medications, like steriods or some antibiotics, can weaken the tendon and contribute to ruptures. 

Signs and Symptoms of an Achilles Tendon Rupture
When a rupture occurs, the patient may experience sudden pain in the back of the ankle or calf, a popping or snapping sensation, swelling in the back of the leg between the heel and calf, difficulty rising up on the toes, or difficulty walking up inclines (stairs, ramps, etc.)
These symptoms should be taken seriously and require prompt medical attention to prevent further damage. Until you are able to see your podiatrist, the R.I.C.E. method should be used:

R est- Stay off the injured ankle as much as you can, as walking can further aggravate the pain or cause further damage. 
I ce- Apply a bag of ice covered in a thin towel to reduce the swelling and pain. DO NOT put ice directly against the skin.
C ompress- Wrap the foot and ankle in an elastic bandage to prevent further swelling.
E levate- Keep the leg elevated to be even with or slightly above your heart. This will help reduce swelling.

Diagnosis
On seeing the doctor, he or she will ask how the injury occured and if you've had a similar injury before, or had similar symptoms. The podiatrist will examine your foot and ankle, testing range of motion and muscle strength in comparison to the uninjured foot/ankle. While the diagnosis of Achilles Tendon Ruptures can usually be made in this way, the podiatrist may order an MRI or other imaging tests to aid in the diagnosis.

An image of a normal Achilles tendon                                                       An image of a ruptured Achilles tendon