From Joomla! Documentation
The Template Manager is where you assign a default Joomla! Template to your Joomla! web site. You can also edit and preview Templates here.
In Joomla!, the visual layout of both the Front-end and Back-end of your site is controlled by the Template. Templates are extensions that contain layout and style information that tells Joomla! exactly how to draw each page of your site.
When you first install Joomla!, one Back-end Template, see Administrator, and two Front-end templates are included. Other Templates can be installed from third-party developers as Extensions.
If you want to use the same Template for all of the pages on your site, you just assign one Template as the Default Template. You can also assign different Templates to different pages.