From Wikipedia: 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.
As you have read the most important thing in Rest architecture is the existance of a resource. This resource can be anything (typically required information requested by client) that can be identified with a global identifier (URI in case of HTTP). In order to manipulate these resources, client communicates using standard interfaces (like HTTP) and exchange representations of these resources (using HTML, XML, ...).
Note that Rest does not force you to use any specific network protocol nor how resources are identified.
For those who have never read about Rest this description of Rest architecture could seem something strange and bit complicated.
A RESTful web service is a simple web service implemented using HTTP and the principles ofREST. URI is defined as global identified, communication interface is HTTP and resource representation can be any valid Internet media type like JSON, XML or YAML. The set of operations that can be executed to resources depend on HTTP Methods and are (GET - retrieving/listing, PUT - replacing/updating, POST - creating and DELETE - deleting).
Let's create our first Rest application with help of Spring MVC. Imagine an application that has a database of manga characters, and you want to provide a Rest interface so clients can retrieve characters following a RESTful strategy.