REST is a software architecture style, defined by Roy Fielding in his PhD thesis (2000). There is a multitude of information on RESTful design principles, development frameworks and examples. For a start, the following references could be recommended:
Contrary to established WS-SOAP standards, there are no (currently) standards for RESTful applications, but merely design guides. Perhaps the first move towards standardization is the First International Workshop on RESTful Design WS-REST 2010.
REST style can be briefly summarized as:
Every object (resource) is named and addressable (e.g. HTTP URL ) Examples:
RESTfull API design starts by identifying most important objects and groups of objects, supported by the software system and proceeds by defining URL patterns. Common patterns are:
HTTP is the most popular choice of transport protocol, but there are examples of systems using other protocols as well.
Resources (nouns) support limited number of operations (verbs). HTTP operations are the common choice, when the transport protocol is HTTP.
Deciding on the operation to be done, on the basis of interpreting POSTed message content (the way a typical SOAP service works!) is NOT recommended. This is referred to as "overloaded POST" and considered violation of RESTful principles!
All operations, except POST should be safe (no side effects) and idempotent (same effect if executed multiple times).
Every resource is defined by an URI. If GET operation on a resource URI returns some content, it is regarded as "dereferencable" (effectively it is an URL). A resource may return content in different formats (by specifying MediaType in the Accept: header of GET operation). The content is regarded as resource "representation". There could be multiple representations of the same resource (e.g. text/html or text/xml).
RESTfull API design includes specification of allowed media types for each resource/operation pair.
All resources should be reachable via a single (or minimum) number of entry points into a RESTful applications. Thus, a representation of a resource should return hypermedia links to related resources
RESTful design principles are advocated as being successful, for underlying the existing WWW architecture. REST application are becoming increasingly popular, the trend with major service vendors are to offer REST API along with an existing SOAP API. Some report REST API usage is increasing and SOAP API usage decreasing.
Non exhaustive list of popular frameworks for developing RESTful applications: