The standard distribution is compiled with Tcl, which add a complete scripting language including variables, loops, and conditionals along with a standard method for communicating with other programs via standard TCP/IP sockets. Versions 1.2 and later also include the Tk toolkit, for creating menus with buttons bound to one's favorite actions.
Tcl (short for Tool Command Language, developed by John Ousterhout) is an embeddable and extensible scripting language. In other words, Tcl sits inside VMD as a language interpreter where it can execute its standard language commands or the various VMD specific extensions.
VMD uses Tcl and Tk version 8.4.1. We refer you to http://www.tcl.tk/ for more information about Tcl.