A CMS (content management system) is a system that allows you to maintain your own website in an easy and effective way. A CMS makes it possible to add your own news, texts or photos.
Because a CMS uses databases and programming languages like PHP it is important your webhosting company supports this. Most do, but if you aren’t sure you can contact your webhosting company about this.
JOOMLA!

Joomla! is one of the most powerful and often used open source content management systems. It is easy to install and very reliable. Joomla is especially suitable for larger sites or sites that run in combination with a web shop.
The entire content is divided in sections and categories, a so-called structure that makes it very clear to handle a site with a lot of information. If you only need a few pages Joomla! can be a bit too extensive.
WORDPRESS
WordPress is a system originally developed for weblogs. Nowadays WordPress is used more and more for business websites.
It is easy to use, and quick to install. With the use of plugins the possibilities are unlimited, it is even possible to integrate a webshop.
Which system is best for you, depends on your wishes and the websites function. Functionally both systems are excellent but one system might fit better with the content and purpose of your website.