Hmm, CMS.
Stands for Content Management System. A little like a free site builder with huge amount of extras and that's saying something.
Theres a lot of variations of course, Joomla being one of them along with the two that I usually recommend; PHPNUKE and E107
http://www.phpnuke.com/
http://www.e107.org/
Basically to install them you need to have a host that is compatible with PHP 4 + and MySQL databases compatible make sure to take note once you create them!
First you need to download them upload them, run the scripts (installation entering database information when prompted) and off you go you have your set-up CMS.
From then on its customization all the way, you have the power to choose templates that people have made integrate plugins that people have made that add extra features alongside the built in tools. Some say its the all in one package and I completely agree.
Its obviously completely free, and includes a page builder, template chooser. SEO optimization such as meta tag and descriptions keyword distributions and lots more that you can get your hands on it really is so accessible and easy to use.
Some of the main features are free built in forums, RSS syndicates and gallery's. You can activate these features when you want a control what they do fully, also if you are using FTP you can always download some files and modify the look or function via a HTML editor and add your own bits and re-upload.
Definitely worth a try explore it press every button and find out everything and then decide whether you like it; its an awesome creation and so easy to use. Heres the official WIKI resource if you want to find out even more...
http://en.wikipedia.org/wiki/Content_management_system