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 quick-start introduction in scripting, see Cameron Laird's column in SunWorld, called "Regular Expressions", which kicked off with this article:
Look for more info about Tcl/Tk at:
WiKit, the software for this wiki, is also written in Tcl/Tk, as is Wub, the webserver that hosts the wiki.