The GTK5.1 is one of many X Window System programming toolkits available now. The GTK (the GIMP Toolkit) was originally developed as a toolkit for the GIMP (General Image Manipulation Program). The GTK is built on the top of the GDK (the GIMP Drawing Kit) which is basically a wrapper around the Xlib functions. The Xlib is a basic library of the X Window System - graphics oriented visualization system for Unix environments.
The GTK is essentially an object oriented application programmers interface (API). Although written completely in C, it is implemented using the idea of classes and call-back functions (pointers to functions).
There is also a third component called the GLib which contains a few replacements for some standard calls, as well as some additional functions for handling linked lists etc. The replacement functions are used to increase GTK's portability.
The GTK is designed to be accessible from different languages, there are bindings for C++, Objective C, Perl, Python, Scheme, Ada and Prolog.
More information about GTK can be found in Appendix B, at the GTK's web-site: www.gtk.org and in [9,10,11].