Joomla! CMS is the 2nd most popular content management system powering the internet according to some statistics, WordPress being the most popular, and Drupal the third.
Like both competitors it is open source, PHP based, and requires a server (e.g. Apache), MySQL or MariaDB database, PHP and Perl. And similar to both it can be extended easily with plug-ins and themes of which there are thousands.
The main difference between the three systems is in the management structure behind the projects (very much driving by a single person or small core of people with Drupal and WordPress, less obvious with Joomla) and commercial focus (much more pronounced with WordPress and Joomla, with many more plug-ins or modules being commercial) than with Drupal..
The administrative user interface tends to be beautifully designed but more confusing than WordPress due to the often very large number of options and the sometimes distracting ‘candy store’ styling of too many icons, but has a much lower learning curve than the usually more bare bones and ‘you have to edit this or that file’ Drupal approach.
My first CMS driven website was Joomla based, but I have since moved on to Drupal and WordPress for various reasons, not the least of which was the frequency with which plug-ins caused problems due to insufficient testing prior to release at that time. This certainly will have improved with more recent Joomla releases.
Joomla! can be downloaded and plugins and themes browsed here: