REST is an architectural style, a set of constraints applied to the design of a web API. An API can be considered RESTful if it satisfies ALL of the constraints.

The most commonly ignored/misunderstood constraint is that of Hypermedia As The Engine Of Application State. You can read more about the application of the term here: http://roysvork.wordpress.com/2013/03/13/why-im-giving-rest-...