Tcl is the programming language that is used to implement, configure and extend exmh. Tk is the widget toolkit, integrated closely with Tcl, that displays exmh's graphical user interface.
A wealth of information about Tcl/Tk is available starting from the easy-to-remember URL:
In addition, there's a rich Wiki about Tcl programming at
which includes introductory material at
Brent Welch, host of this website, is the author of Practical Programming in Tcl and Tk [1]. Several chapters from various editions are available for free download, including Chapter 1 for a basic introduction [2] (pdf)