The GIMP ToolKit (GTK+) is a collection of GUI widgets.  GTK+
essentially provides the building blocks from which GUIs can be
built.  It is highly themable, and its functionality is highly
extensible.

GTK+-3 is a very stable release, similar only in design to GTK+-2.
GTK+-3 can coexist happily alongside GTK+-2, but applications are
written for one version or the other.