Representational State Transfer is a term that sounds very academic. It is best explained using the web where it is put to use every time you use the web. A typical web interaction involves a user in front of a web browser ( the client ) making a request to say get some resource from a server by typing in a URL. The representation of the resource is a document. The location of the document is described by the URL. Getting the resource from the server and displaying in the browser causes the state transition in the browser.
The protocol used in the web browser-server communication is HTTP described in RFC2616.
When you type the URL http://www.xyz.com/path/mydoc.html, the browser requests the document from the host by sending the GET command.
Everyone has at some point filled out a form on a web page to may be open an account or to buy something online. When you click the submit button, the browser sends the data to the server using the http POST command.
Other common http commands are PUT which is used to update a document on the server and DELETE which is used to delete a document from the server.
This sort of interaction does not necessarily have to take place only between a browser and a server. It could very well be two servers or applications exchanging documents or data. The term web services is generally used to describe this kind of interaction. The two applications could have been written in different programming languages. But that does not matter since they are talking HTTP. Ironically, web services were popularized by SOAP/WSDL, the technology that REST is displacing.
SOAP based web services use XML to describe the operation or method to be invoked on the server, the data that is passed as input and the data that is output. In SOAP, describing the interface, operation and data has to done for each and every application. For example, for an application that manages users, you define methods like getUser, createUser, deleteUser and so on. For an application that manages accounts, you might define methods like getAccount, createAccount, unaccountably and etc.
Contrast this with REST , where the operations stay the same no matter what the application is - GET, PUT, POST, DELETE. The resources are identified by URLs such as http://www.xyz.com/user/userid or
http//www.xyx.com/account/accountid. REST is thus a lot simpler. Data is transferred as the body of the HTTP message. Popular formats are XML and JSON.
An example of a GET request is
GET /account/A12345 HTTP/1.1
and the server response could be
HTTP/1.1 200 OK
Content-Type: text/json; charset=utf-8
Similarly to create a new account, the request would be
POST /account HTTP/1.1
An important characteristic of RESTful applications is that they are stateless. Each request has all the information necessary for the server to respond to the request. This is a critical requirement for such web services applications to be highly scalable. By being stateless, each subsequent request can be serviced by any other peer server, which means you can use a cluster of servers to service requests and you can have high availability and failover.
In summary, REST is a simple and powerful architectural style for building web services. REST requires identifying the data or resources in your application using URLs. The HTTP commands such as GET,PUT,POST,DELETE are the verbs or operations exposed by the application. The client and server communicate using the HTTP protocol with the data carried in the body of the message.