From Joomla! Documentation
The Site application, also known as the Frontend, is the main part of a Joomla website. This is where the general traffic occurs. When visitors come to your site, they will be using the Site application. You can manipulate this (add/remove content, switch templates etc.) from the Administrator application. Users with appropriate access levels will also be able to edit some aspects of the site from the Frontend.
The Site application is implemented in the JSite class, which is a subclass of the generic JApplication. The JSite object can be retrieved in all versions that support JSite using the following code. The variable $app will be a JSite object when this code is run in the Site application.
$app = JFactory::getApplication();