From Joomla! Documentation
This page contains many links to selected documentation concerning Template Development for and . A good place to start is with the Recommended Reading articles below as they provide a good introductory base of knowledge to build on.
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.