In the REST architectural style, data and functionality are considered resources and are accessed using Uniform Resource Identifiers (URIs), typically links on the Web.
The resources are acted upon by using a set of simple, well-defined operations. The REST architectural style constrains an architecture to a client/server architecture and is designed to use a stateless communication protocol, typically HTTP.
Java EE 6 introduced new package to deal with RESTful webservices named JAX-RS.
Developing RESTful webservices by JAX-RS very simple with simple annotations like @PATH, @GET, @PUT, @POST, @DELETE, etc.
Refer Oracle's website and study JAX-RS tutorials, you will gain knowledge in RESTful webservices.