One of the constraints defined for the architectural style known as REST is "hypermedia as the engine of application state". In a new InfoQ article, Mark Baker, well-known for being one of the first who advocated the REST style instead of the mainstream web services approach, discusses what the hypermedia constraints means in practice and why it is essential to RESTful design.
According to Mark, the most important aspect of hypermedia is the usage of standardized identifiers - URIs in case of the Web - instead of proprietary identification protocols, and the application model that is enabled by them:
By using a common application model, one that is not just standardized, but fixed for all time, you are reducing coupling between consumer and producer by permitting each to evolve independently of the other. This way, old and new services can be combined together into a composite application, and old and new clients can be the ones doing that combining. I suppose we take it for granted in our use of the Web that one can simply include a link in a document to a page authored years ago and a consumer of that content can seamlessly navigate between them without having to download a new version of the browser. That's by design, not by accident.
Read the full article for more.