"Tcl stands for Tool Command Language. Tcl is really two things: a scripting language,
and an interpreter for that language that is designed to be easy to embed into your application.
Tcl and its associated graphical user interface toolkit, Tk, were designed and crafted by Professor John Ousterhout
of the University of California, Berkeley. You can find these packages on the Internet and
use them freely in your application, even if it is commercial. The Tcl interpreter has been ported
from UNIX to DOS, Windows, OS/2, NT, and Macintosh environments.
The Tk toolkit has been ported from the X window system to Windows and Macintosh.
There are many Tcl extensions freely available on the Internet.
Most extensions include a C library that provides some new functionality,
and a Tcl interface to the library. Examples include database access, telephone control,
MIDI controller access, and expect, which adds Tcl commands to control interactive programs.
The most notable extension is Tk, a toolkit for graphical user interfaces.
Tk defines Tcl commands that let you create and manipulate user interface widgets." (Brent Welch)
"[incr Tcl] provides the extra language support needed to build large Tcl/Tk applications.
It introduces the notion of objects, which act as building blocks for an application.
Each object is a bag of data with a set of procedures or "methods" that are used to manipulate it.
Objects are organized into "classes" with identical characteristics, and classes can inherit functionality from one another.
This object-oriented paradigm adds another level of organization on top of the basic variable/procedure elements,
and the resulting code is easier to understand and maintain." (WebNet Technologies)