There are two types of Achilles tendonitis, depending on which part of the tendon is affected. The non-insertional type occurs when the middle part of the tendon begins to tear and degenerate, becoming swollen and thick. Insertional involves the lower portion that attaches to the heel bone. Damaged tendon fibers can calcify, and extra bone growths, called spurs, can add to the painful condition.
If your Achilles is hurting, do not ignore it in hopes that it will eventually go away. It will only worsen and possibly even rupture.