A calcaneal spur is a small osteophyte located on the calcaneus. When a foot bone is exposed to constant stress, calcium deposits build up on the heel bone.

Inferior calcaneal spur is located on the inferior aspect of the calcaneus and is typically a response to plantar fasciitis.

A posterior calcaneal spur develops on the back of the heel at the insertion of the achilles tendon.[1]

 

Reference:

[1] Wikipedia. (n.d.). Calcaneal Spur. Retrieved 24 April 2013 from http://en.wikipedia.org/wiki/Calcaneal_spur