The core librairies provide common functionnalities to the Xfce environment. There are three libraries which provides non-graphical helper functions, widgets and convenience functions for graphical user interfaces, and functions for settings management.
svn co http://svn.xfce.org/svn/xfce/libxfce4util/trunk libxfce4util
svn co http://svn.xfce.org/svn/xfce/libxfcegui4/trunk libxfcegui4
svn co http://svn.xfce.org/svn/xfce/libxfce4mcs/trunk libxfce4mcs