From Joomla! Documentation
A template is a type of Joomla! extension that changes the way your site looks. There are two types of templates used by the Joomla! CMS: Frontend Templates and Backend Templates. The Frontend Template controls the way your website is presented to the user viewing the website's content. The Backend Template controls the way your website's administrative tasks are presented for controlling management functions by a Joomla! Administrator. These include common tasks such as user, menu, article, category, module, component, plugin and template management.