XF handles all low-level including initialization, database connection and basic abstraction layers
any 'action' inherit XFAction must define at least some common methods/vars...
XFCache provides an abstraction for caching organized in pools
XFDebug handles all debugging purposes and requires XF::DEBUG to be 'true'
well, XFE is my little own interpretation of a dump after catching an exception.
basic elements to inherit by XFTask for scheduled tasks either...
XFUI class handles user interface methods