Joomla is a Content Management System similar to WordPress.
Basically, CMS use databases to arrange content that you create and present it into a website when a page is requested.
The biggest different between WordPress and Joomla is that WordPress is post-centric and Joomla is menu-centric. What this means is that everything from Joomla ‘hangs’ off of a menu item on your site, whereas WordPress filters posts based on what you are looking for.