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From Joomla! Documentation
The Template controls the overall look and layout of your site and how your site would appear to people. It provides the framework that brings together common elements, modules and components as well as providing the cascading style sheet for your site. Both the Front-end (Site) and the Back-end (Administrator) of your site have templates.
When Joomla! is installed several templates are automatically included. You can find many more templates at other websites or create your own according to Joomla! standards. Some are available without charge under various licenses, and some are for sale. In addition, there are many designers available who can make custom templates. You can also make your own template.