It stands for Extensible hypertext markup language and is an application of XML. Most Joomla pages are based on XHTML.

The template used to specify the layout of the administrator back end of a Joomla site. 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.