From Joomla! Documentation
The Administrator application, also known as the Back-end, Admin Panel or Control Panel, is the interface where administrators and other site officials with appropriate privileges can manipulate the look of a Joomla! powered web site. There are many tasks which can be done with the administrator interface. You can set up how your web site looks with the Template Manager or completely change a web site's look by changing the template to another template. You can also add new extensions such as components, languages, modules, and plugins. Many of these are found on the Joomla! Extension Directory (JED).