From Kai Michelsen:
Tcl (Tool Command Language) is two things: a scripting language and an interpreter for that language that is designed to be easy to embed into other applications. As a language, it is much like UNIX shell languages. There is very little syntax and it is easy to learn. It is used to glue together building blocks built in system programming languages like C, C++, and Java. These building blocks appear as commands, or verbs, in the script language. It is easy to embed Tcl into a legacy program so you can script the behavior of that program and add in other building blocks like a GUI interface. The Tcl interpreter is written in C and has been ported to almost every computer platform. Here is an introductory chapter on Tcl programming.