From Joomla! Documentation
This page contains many links to selected documentation concerning Template Development for and . A good place to start is with the Recommending 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: Front-end Templates and Back-end Templates. The Front-end Template controls the way your website is presented to the user viewing the website's content. The Back-end Template controls the way your website's administrative tasks are presented for controlling management functions by a Joomla! Administrator. These would include common tasks such as: user, menu, article, category, module, component, plugin and template management.