The Story of REST: Representational State Transfer

Joe Gregario has provided a lucid description of REST at YouTube.

He has also provided a companion video on the Atom Publishing Protocol, which is described in the Wikipedia article on Rest is described as a canonical RESTful protocol. So Atom provides a great example of a real-world system that uses REST and Joe describes it clearly.

If you want a clear understanding of how to use Rest, this may be your best use of half an hour. If all you want to do is to grok Rest, Ryan Tomayako's How I Explained RESR To My Wife is a classic, and it only takes five to ten minutes.

If you have more time and really want to understand in greater detail, there is Roy Thomas Fielding's dissertation, Architectural Styles and
the Design of Network-based Software Architectures.

For an actual implementation of a REST service, I am keen to use Grails. Grails has build-in support for REST. Grails also offers content negotiation. In HTTP, part of the request is a specification of the media type. Using content negotiation, the user agent can specify which format it prefers. This means that a URL for a person could return a portrait as image/jpeg, a hcard page via text/html, the vcard data as text/json, text/plain, text/xml, or text-plain. You could also use content negotiation to specify the language for the response. Suddenly URL seems to be the locator for a universal resource, as well as being the universal locator of a resource.

While it may be feasible to support multiple mime types at a single URL, it is not trivial to provide multiple representations of the same item. Even converting between two apparently similar data format has surprising complexity, as discussed in
Convert Atom documents to JSON at IBM Developer Works. The Developer Works also has a series of papers on Grails, which has specific examples of using Grails with Atom syndication.