From: Roy T. Fielding

I am getting frustrated by the number of people calling any HTTP-based interface a REST API. Today’s example is the SocialSite REST API. That is RPC. It screams RPC. There is so much coupling on display that it should be given an X rating.

Source: http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypertext-driven
More: http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm