Tcl stands for Tool Command Language. It is a powerful but easy to learn programming language. Tk is a GUI interface Toolkit to use with Tcl.
Tcl is an open source very powerful but easy to learn dynamic programming language. It is suitable for a very wide range of uses, including web and desktop applications, networking, administration, testing and many more. Open source and business-friendly, Tcl is a mature yet evolving language that is truly cross platform, easily deployed and highly extensible. It is well integrated with SQLite.
Tk is a graphical user interface toolkit that takes developing desktop applications to a higher level than conventional approaches. Tk is the standard GUI not only for Tcl, but for many other dynamic languages, and can produce rich, native applications that run unchanged across Windows, Mac OS X, Linux and more. When used with tcl it is called tcl/tk.