A user level on a Joomla site that is the same a Public except for the Allow permission for the Site Login action. This means members of this group have access only to the front-end access to the site. Should you wish it, a registered user is able to see additional content to that seen by casual visitors to your site.

Adjusting a core Joomla file (i.e., the files that you uploaded before installing Joomla!). Any changes you make to core files will be overwritten during the next update, thus core hack is not recommended. In most cases, there are alternative workaround to change Joomla without hacking the core.