PLT Redex consists of a domain-specific language for specifying reduction semantics, plus a suite of tools for working with the semantics.
This is a reference manual for Redex. See http://redex.plt-scheme.org/ for a gentler overview. (See also the examples subdirectory in the redex collection.)
The module redex/reduction-semantics provides only the non-GUI portions of what is described in this manual (everything except the last two sections), making it suitable for use with mzscheme scripts.