It is a useful widget set complementary to GTK+ for creating graphical interfaces for the X Window System. It is licensed using the LGPL license, so you can develop open software, free software, or even commercial non-free software using GTK without having to spend anything for licenses or royalties.
GtkExtra is essentially an object oriented application programmers interface (API). Although written completely in C, it is implemented using the idea of classes and callback functions (pointers to functions).
This document is a "work in progress". Please look for updates on CVS at http://gtkextra.sourceforge,net/.
I would very much like to hear of any problems you have learning GtkExtra from this document, and would appreciate input as to how it may be improved. Please see the section on Contributing for further information.