Tcl is an interpreted scripting language, while Tk is the graphical toolkit that extends Tcl with the possibility to create graphical user interfaces. The combination forms a powerful tool for prototyping. Because Tcl/Tk is interpreted, it's possible to edit a program while it's being executed, thereby providing a means to quickly develop a prototype.
This document will give an introduction to Tcl and Tk and will show how Tcl/Tk can be used for creating a simple prototype.