In his 2000 dissertation Roy Fielding coined the term REST for Representational State Transfer to describe an architecture style of networked systems. A simple description of REST is that it is a method for acquiring information content from a web site by reading a designated web page that contains an Extensible Markup Language (XML) file that describes and includes the desired content.
Here are two excellent resources that describe REST in more detail:
The Blog: Learn REST a Tutorial
Roy’s Dissertation: Architectural Styles and the Design of Network-based Software Architectures