What is REST?
The concept of REST (REpresentational State Transfer) was introduced by Roy Fielding in 2000, as a part of his doctoral dissertation. Basically, he described an architectural style that exploits the existing technology and protocols of the Web, including HTTP and XML. So, we can define REST as a scalable architecture for building services on a uniform interface(HTTP) focused on resources.
A resource can be anything from IT infrastructure elements such as servers, computers and includes web pages, scripts and other entities as well.
REST is considered to be a lightweight alternative to mechanisms like RPC (Remote Procedure Calls) and Web Services (SOAP, WSDL). Some of the popular REST API services are used by Google (Google Glass API), Twitter, Flickr and Atom. Despite being simple, REST is fully featured and supports everything that Web Services can do.