GTK+ is the widget toolkit written in C originally developed for the GIMP (www.gimp.org) and now used as a general windowing toolkit which serves as the foundation the GNOME project as well as many stand-alone applications. GTK+ is the foundation gtkmm is built on. If you want to work with gtkmm, taking a look at the materials provided at the documentation and FAQ provided at http://www.gtk.org is probably a good idea.