Tcl is a scripting language and Tk is a graphical widget set. They are made to work together very nicely to provide functionality from simple interfaces for your scripts to full fledged graphical applications. Tcl/Tk is probably the single most portable programming language in existence. I stopped counting supported platforms at 23. And that number has all Linux distros lumped into one. Suffice it to say, if the platform has any popularity at all, Tcl/Tk will be supported there.

Let's Play

A simple interactive gui-building session 

$ wish
% button .submit -text "Click Me" -command { puts "\nHello World" }
% pack .submit

If you really want to develop any GUI of reasonable size or complexity, I highly suggest trying Visual Tcl. It is quite an amazing program. The entire thing is written in Tcl/Tk, so if you have a basic knowledge of programming it's pretty easy to see how nice it would be to write a GUI Builder for this language in this language. It's rather impressive.

Links
A Good Tcl/TK Hands-On Tutorial
Visual Tcl Tutorial