Excerpt from Wikipedia
The Achilles tendon (or occasionally Achilles’ tendon), also known as the calcaneal tendon or the tendo calcaneus, is a tendon of the posterior leg. It serves to attach the plantaris, gastrocnemius (calf) and soleus muscles to the calcaneus (heel) bone.
If you have Achilles Tendinosis here is an informative Wikipedia page that gives you an overview of your treatment options:
http://en.wikipedia.org/wiki/Tendinosis