I think the best definition is in Fielding's thesis where he lists the 4 REST interface constraints: 1) Identification of resources 2) Manipulations through rep

I think the best definition is in Fielding's thesis where he lists the 4 REST interface constraints: 1) Identification of resources 2) Manipulations through representations 3) Self-descriptive messages and 4) HATEOAS. 1 & 2 mean that you should name a resource something for identification only and that name should be used no matter what you are doing to it. In HTTP you use a combination of verbs and request body data to tell the server what to do with a resource. HTTP implements #3 though status codes including verbose descriptions in the response body. HATEOAS (Hypermedia as the engine of application state) means that 100% of functionality must be accessible through a browser.