From Joomla! Documentation
Revision as of 17:59, 29 January 2011 by Mvangeest
Core hacking is not attempting to force your way into the Joomla! core, but adjusting the Joomla! core files (i.e., the files that you uploaded before installing Joomla!), which should actually be hidden from and left alone by users and extension developers. Any change you make to core files will be overwritten during the next update.