Before going into the specifics of the GNOME libraries, it is important to see where do they fit in the picture of all the different libraries that are used in a GNOME application. The GNOME libraries are the most high level. GTK+ with it's two parts, GTK and GDK, comes next. GTK level provides an object model for C and a UI toolkit with the basic widgets to provide the generic basis for a GUI. GTK depends on GDK, which is a low-level wrapper around Xlib, the library directly talking to the X server. Everything (except for Xlib) depends on GLib which is a very useful C library with many utility and portability functions as well as a range of easy to use containers for C.
Figure 1-1. GNOME Application Library Hierarchy