REST is a term coined by Roy Fielding in his Ph.D. dissertation to describe an architecture style of networked systems. REST is an acronym standing for Representational State Transfer.
I understand that this stuff may sound abstract, but it is useful (and practical).
This page gives a good summary of what REST is.
While this page gives you the actual dissertation by Roy Fielding, which he calls "Architectural Styles and the Design of Network-based Software Architectures"
REST is fundametally useful... because it is a "lightweight" approach, as compared to heavier models such as SOAP. In fact, it is much simple to implement REST than SOAP.