This is GTK+.   GTK, which stands for the Gimp ToolKit, 
is a library for creating graphical user interfaces.  

In this directory you will find:

binary/    Binary packages 
contrib/   Contributed packages and widgets
gtk--/     C++ bindings
perl/      Perl bindings
python/    Python bindings
tutorial/  The GTK tutorial
v0.99/     Old Development Series GTK+ 0.99
v1.0/      Old Stable Series GTK+ 1.0
v1.1/      Old Development Series GTK+ 1.1
v1.2/      Old Stable Series GTK+ 1.2
v1.3/      Old Development Series GTK+ 1.3
v2.0/      Old Stable Series GTK+ 2.0
v2.1/      Old Development Series GTK+ 2.1
v2.2/      Old Stable Series GTK+ 2.2
v2.3/      Old Development Series GTK+ 2.3
v2.4/      Old Stable Series GTK+ 2.4
v2.5/      Old Development Series GTK+ 2.5
v2.6/      Old Stable Series GTK+ 2.6
v2.7/      Old Development Series GTK+ 2.7
v2.8/      Old Stable Series GTK+ 2.8
v2.9/      Old Development Series GTK+ 2.9
v2.10/     Current Stable Series GTK+ 2.10
2.11/      unstable 2.11 development series
2.12/      stable 2.12 series
2.13/      unstable 2.13 development series
2.14/      stable 2.14 series
2.15/      unstable 2.15 development series
2.16/      stable 2.16 series
2.17/      unstable 2.17 development series
2.18/      stable 2.18 series
2.19/      unstable 2.19 development series
           and so on...

Note that starting with v2.10, GLib and Pango
have been moved to their own separate directories,
  ftp://ftp.gtk.org/pub/glib
  ftp://ftp.gtk.org/pub/pango
  
The official ftp site is:
  ftp://ftp.gtk.org/pub/gtk

The official web site is:
  http://www.gtk.org/gtk

A mailing list is located at:
  gtk-list@gnome.org

To subscribe: mail -s subscribe gtk-list-request@gnome.org < /dev/null
(Send mail to gtk-list-request@gnome.org with the subject "subscribe")