“Representational State Transfer (REST) is a style of software architecture for distributed hypermedia systems such as the World Wide Web. The term originated in a 2000 doctoral dissertation about the web written by Roy Fielding, one of the principal authors of the HTTP protocol specification, and has quickly passed into widespread use in the networking community.” (Wikipedia, retrieved 12:51, 12 March 2007 (MET)).
REST is popular with the XForms/XQuery XML communities. E.g. see XRX for an XML web development framework that includes REST or the EXist XML database that includes a REST API.
According to Wikipedia, REST only refers to a collection of architectural principles. The term is also often used to describe any simple interface that transmits domain-specific data over HTTP without an additional messaging layer such as SOAP. These two meanings can conflict as well as overlap.