A RESTful Web Service is a web service that implemed the REST architectural style.
The RESTful web API defines a set of standards to interact with resources on the web:
URI (Uniform Resource Identifier)
Identifies in a unique way a resource on the web (ex. http://mysite.com/resources/resource_name ).
Internet Media Type of the data (ex. XML, HTML, JSON).
HTTP Methods (request methods)
Standard methods that can be applied to the specified resource:
GET (requests a specific representation of a resource)
PUT (create or update a resource with the supplied representation)
DELETE (deletes the specified resource)
POST (submits data to be processed by the identified resource)