GTK is a library for creating graphical user interfaces similar to
the Motif "look and feel". It is designed to be small and efficient, but
still flexible enough to allow the programmer freedom in the interfaces
created. GTK allows the programmer to use a variety of standard user
interface widgets such as push, radio and check buttons, menus, lists
and frames. It also provides several "container" widgets which can be
used to control the layout of the user interface elements.

   GTK provides some unique features. (At least, I know of no other
widget library which provides them). For example, a button does not
contain a label, it contains a child widget, which in most instances
will be a label. However, the child widget can also be a pixmap, image
or any combination possible the programmer desires. This flexibility is
adhered to throughout the library.

WWW: http://www.gtk.org/