Joomla! is an open source multifunctional CMS, Meaning it's functionality is similar to Wordpress and Drupal. Being an open source project, Joomla! enables developers to dig into the code and change it to their own advantage. This means that just like the 2 CMS' above, basically anything you think of building, can be built with Joomla!.
Some of the biggest vendors in the market sell top rated templates for Joomla! sites. Below you can see some of the top ones.