This fast and versatile library is used all over the world for all
libgnome (GNOME) applications, gimp (The GIMP), and several others.
Originally, version 1 was written for the GIMP and hence has the name
GIMP ToolKit. Many people like it because it is small, efficient, and
very configurable.

Most applications which used gtk (GTK1) have since switched to GTK2.
Besides a more flexible API, it provides improved text rendering using
pango (Pango) and many other goodies, but GTK+ 2 lacks compatibility
with GTK1, so to switch, programmers have to port applications to it.

To develop applications with gtk, you need the package gtk2-devel.