What is Joomla?
Joomla is a free, open source CMS (content management system) and similar to Wordpress - but better, in my opinion. It was initially created by developers who were frustrated with the limitations of Wordpress. While Wordpress has improved in the last few years, I still prefer to use Joomla.
While both platforms are fine for most websites, I have found Joomla to be more robust and better supported. It offers better extensions and has a more intuitive back-end administrative interface.
The differences between the two platforms are a lot like the argument of "Windows vs Mac". Ask anyone which they prefer, and they'll insist that one is better than the other when in reality both are fine - it just depends on what the user is more comfortable with.
CMS websites are popular with clients who wish to manage their website content themselves. Nearly all updates can be done without the need for the client to have HTML coding experience.