Extended Tcl is a superset of standard Tcl. Extended Tcl has three
basic functional areas: A set of new commands, a Tcl shell (a Unix
shell-style command line and interactive environment), and a
user-extensible library of useful Tcl procedures, any of which can be
automatically loaded on the first attempt to execute it.

In addition, a detailed help system is available for Tcl/Tk: tclhelp.