I see more and more blogs using the word REST which stand for Representational State Transfer, but what the heck does this means??
Did my googling and found that the REST architecture was first described in this paper a very good read indeed.
Here is what I’ve learnt from it. If you consider the World Wide Web as one huge distributed application, REST is the architecture for this application.
WWW being such a successful distributed application it is very interesting to put a layer of abstraction on its architecture. This enables us to extract the main properties of the World Wide Web architecture and understand why it is so good.
First the WWW protocols are mostly stateless => which is good for scalability
WWW resources are cacheable => performance
WWW resources are available through one generic interface => better interoperability and ease of extension.
WWW has a layered architecture => decoupling of components, ease of maintenance and evolution

Examples of REST API include:
http://www.bloglines.com/services/
http://del.icio.us/doc/api
http://www.flickr.com/services/api/request.rest.html

Three very successful services.