REST (Representational State Transfer) is an architectural style that uses the identification of resources; Manipulation of resources through representations; Self-descriptive messaging, and hypermedia, as the engine of application state, to create distributed systems that are scalable and resilient to change.
The term was introduced and defined in 2000 by Roy Fielding in his doctoral dissertation. Fielding is one of the lead authors of the Hypertext Transfer Protocol (HTTP) specification versions 1.0 and 1.1.
In simple language, REST is an alternative to SOAP based web services. Where SOAP tries to model the exchange between client and server as calls to objects, REST tries to be faithful to the web domain.