See also: Extension types (general definitions)
There are two types of templates used in a Joomla CMS powered website.
Front-end templates change the way your site looks to average users. Most of the templates you use or install will be Front-end Templates.
Back-end Templates are much rarer than Frontend Templates. You will probably never need to change your Back-end Template. Back-end Templates allow you to change the way the administrator interface looks.
Joomla administrators should go to the page on Template Management for topics about managing your Joomla template or templates.
Joomla developers should go to the Template Development portal page for more topics on developing a Joomla template.