Tcl is a scripting language and stands for "Tool Command Language".
Tk is a GUI built on top of Tcl and is an abbreviation for "Toolkit".

For a quickstart introduction in scripting, see Cameron Laird's column in SunWorld, called "Regular Expressions", which kicked off with this article:
   * http://www.sunworld.com/sunworldonline/swol-08-1998/swol-08-regex.html

Look for more info about Tcl/Tk at:
   * http://www.scriptics.com/
   * http://www.tclconsortium.org/
   * http://www.purl.org/net/tcl-faq/

WiKit is written in Tcl/Tk.