Tk is a widget library, that is, a collection of boxes and buttons that you can click on to make the computer do stuff. It is very popular. It is most commonly used in tcl programs, and many very nice projects have been done this way.
Many people are beginning to use Tk via perl, however, as perl is a more powerful language than tcl. Perl is also more widely used for different purposes, which makes integrating widgets easier.
(Update, FvwmConf works with perl Tk800 as well)
Down load Tk400.202.tar.gz, unpack it, and type,
perl Makefile.PL
make
make install
make test