Tcl (Tool Command Language) is a dynamic programming/scripting language based on concepts of Lisp, C, and Unix shells. It can be used interactively, or by running scripts (programs) which can use a package system for structuring, hence allowing to do much with little code. Tcl is available for Linux, Windows, Mac OS X, as well as other platforms, as open-source software under BSD-like license, or as pre-built binaries.
This book also discusses the GUI toolkit Tk in considerable extent, even though it's not in the title. For most users, Tcl/Tk form a single conceptual entity.