REST stands for Representational State Transfer. It’s a software architectural style for Web-based systems providing Web Services. It has become a preferable architecture style because it results in better Web performance, scalability, and simplified client and server interactions.

The resources on the Internet are the documents that can be access through URIs(Uniform Resource Identifier). A RESTful API means to provide the unique access to each document. Since each response will return the same document, when the client sends a request, its browser will check the modified date and it can probably use the cached document. This increases the Web performance on client side and at the same time reduces a great amount of traffic to the server. The other major feature of REST is keep the Web state-less, which means no needs for the session management.

Different from the RPC calls, the URIs for REST API calls are supposed to be used for locating the resource only and not the operations, which should be specified by the type of the HTTP request. There are four major types of requests for RESTful Web services:
1) get – retrieved resources with known URIs
2) post – create or modify resources with known URIs
3) put – create or modify resources. The server will return an URI when allocated
4) delete – remove resources with known URIs

JAX-RS is a Java API for RESTful Web Services. It defines a set of annotations to support HTTP requests. You can create a custom annotation using an @HTTpMethod designator. Here are a list of basic annotations:
@Path – relative URI path in the Web server
@PathParm – bind the value of a URI template parameter
@FormParm – bind the value of a form parameter contained in the request
@Produces – MIME media type that the method will return to the client browser
@Consumes – MIME media type that the method accepts

For more available annotations, please check out Java EE 6 API – javax.ws.rs.

Example:

@Path("/")
public class Library {

 /**
  * Handle get requests to relative path /book
  * @param book ID extracted from the URI
  * @return Book details
  */
 @Path("/book/{id}")
 @Get 
 public Book getBook(@PathParam("id") int id) {...}

 /**
  * Handle post requests that are in json format to relative path /book
  * @param all the form data sent from the client
  * @return Book details
  */
 @Path("/book/{id}")
 @POST 
 @Consumes("application/json")    
 @Produces("application/json")    
 public Book addBook(@FormParam("") Book newBook) {...}

}