Representational State Transfer (REST) is a style of software architecture for distributed systems such as the World Wide Web. REST has emerged as a predominant web service design model. The term representational state transfer was introduced and defined in 2000 by Roy Fielding in his doctoral dissertation. Fielding is one of the principal authors of the Hypertext Transfer Protocol (HTTP) specification versions 1.0 and 1.1.

The REST architectural style was developed by W3C Technical Architecture Group (TAG) in parallel with HTTP/1.1, based on the existing design of HTTP/1.0. The World Wide Web represents the largest implementation of a system conforming to the REST architectural style. REST exemplifies how the Web’s architecture emerged by characterizing and constraining the macro-interactions of the four components of the Web, namely origin servers, gateways, proxies and clients, without imposing limitations on the individual participants. As such, REST essentially governs the proper behavior of participants.

REST-style architectures consist of clients and servers. Clients initiate requests to servers; servers process requests and return appropriate responses. Requests and responses are built around the transfer of representations of resources. A resource can be essentially any coherent and meaningful concept that may be addressed. A representation of a resource is typically a document that captures the current or intended state of a resource. The client begins sending requests when it is ready to make the transition to a new state. While one or more requests are outstanding, the client is considered in transition. The representation of each application state contains links that may be used the next time the client chooses to initiate a new state-transition.

REST facilitates the transaction between web servers by allowing loose coupling between different services. REST is less strongly typed than its counterpart, SOAP. The REST language uses nouns and verbs, and has an emphasis on readability. Unlike SOAP, REST does not require XML parsing and does not require a message header to and from a service provider. This ultimately uses less bandwidth.
REST error-handling also differs from that used by SOAP.

Representational State Transfer is intended to evoke an image of how a well-designed Web application behaves: presented with a network of Web pages (a virtual state-machine), the user progresses through an application by selecting links (state transitions), resulting in the next page (representing the next state of the application) being transferred to the user and rendered for their use. REST was initially described in the context of HTTP, but it is not limited to that protocol. RESTful architectures may be based on other Application Layer protocols if they already provide a rich and uniform vocabulary for applications based on the transfer of meaningful representational state. RESTful applications maximize the use of the existing, well-defined interface and other built-in capabilities provided by the chosen network protocol, and minimize the addition of new application-specific features on top of it.

Vocabulary re-use vs. its arbitrary extension: HTTP and SOAP In addition to URIs; Internet media types; request and response codes; etc., HTTP has a vocabulary of operations called request methods, most notably:
GET
POST
PUT
DELETE

REST uses these operations and other existing features of the HTTP protocol. For example, layered proxy and gateway components perform additional functions on the network, such as HTTP caching and security enforcement. SOAP RPC over HTTP, on the other hand, encourages each application designer to define new, application specific operations that supplant HTTP operations. An example could be:
getUsers()
getNewUsersSince(date SinceDate)
savePurchaseOrder(string CustomerID, string PurchaseOrderID)

This additive, “re-invention of the wheel” vocabulary — defined on the spot and subject to individual judgment or preference — disregards many of HTTP’s existing capabilities, such as authentication, caching, and content-type negotiation. The advantage of SOAP over REST comes from this same limitation: since it does not take advantage of HTTP conventions, SOAP works equally well over raw TCP, named pipes, message queues, etc.

The uniform interface that any REST interface must provide is considered fundamental to the design of any REST service.

• Identification of resources. Individual resources are identified in requests, for example using URIs in web-based REST systems. The resources themselves are conceptually separate from the representations that are returned to the client. For example, the server does not send its database, but rather, perhaps, some HTML, XML or JSON that represents some database records expressed, for instance, in Swahili and encoded in UTF-8, depending on the details of the request and the server implementation.
• Manipulation of resources through these representations. When a client holds a representation of a resource, including any metadata attached, it has enough information to modify or delete the resource on the server, provided it has permission to do so.
• Self-descriptive messages. Each message includes enough information to describe how to process the message. For example, which parser to invoke may be specified by an Internet media type (previously known as a MIME type). Responses also explicitly indicate their cacheability. Hypermedia as the engine of application state (aka HATEOAS)
• Clients make state transitions only through actions that are dynamically identified within hypermedia by the server (e.g., by hyperlinks within hypertext). Except for simple fixed entry points to the application, a client does not assume that any particular action is available for any particular resources beyond those described in representations previously received from the server.

RESTful web services : RESTful web service (also called a RESTful web API) is a web service implemented using HTTP and the principles of REST. It is a collection of resources, with four defined aspects:

• the base URI for the web service, such as http://example.com/resources/
• the Internet media type of the data supported by the web service. This is often JSON but can be any other valid Internet media type provided
• that it is a valid hypertext standard.
• the set of operations supported by the web service using HTTP methods (e.g., GET, PUT, POST, or DELETE).
• The API must be hypertext driven.