From R5RS (5th Revised Report on Scheme):
"Scheme is a statically scoped and properly tail-recursive dialect of the Lisp programming language invented by Guy Lewis Steele Jr. and Gerald Jay Sussman. It was designed to have an exceptionally clear and simple semantics and few different ways to form expressions. A wide variety of programming paradigms, including imperative, functional, and message passing styles, find convenient expression in Scheme."
Peter Norvig has a fascinating comparison of Python and Lisp, where he refers to Python as "Scheme with better libraries."
Scheme is becoming popular as a learning language in high schools and universities. DrScheme is a free implementation of the language, with a complete IDE (written in Scheme) and a complete accompanying text book (How to Design Programs).
More info and other implementations can be found at: http://www.schemers.org/