These examples assume that you already know the basics of C programming. Each one builds a functional program.
GTK is a cross-platform GUI programming library licensed under the GNU LGPL. There is more information on the GTK+ Project page.
Originally written to develop the GNU Image Manipulation Program (GIMP), GTK is the GIMP ToolKit built on top of GDK (GIMP Drawing Kit) which is basically a wrapper for low-level functions used to access the underlying windowing functions (Xlib for Linux).
GTK+ is written in C, but there are bindings for many other languages. We're gonna use C here!. The API is object-oriented using classes and callback functions, most widgets are derived from GObject.
GTK was first released in 1998. GTK+2 has been around since 2002, development ended at Version 2.24 in 2011 with the introduction of GTK+3.