Its basically a web application, like phpbb is. It has a front end and a back end, like phpbb. Instead of only administering forums though, it allows you to administer an entire website through an administration section. As an example, if you look at the jeepsunlimited site, you see all these articles, links, banners and neat stuff. They didn't have to do any html or programming to make that stuff work. They just go into the administration section, type the content into a rich text box, and publish it. It automatically shows up wherever they told it to. It allows you to let users see things that non-users can't, and more.
From there, you can have stuff like arcades, polls, guest books, calendars, pretty much anything you can think of, that just plug into your CMS, with no coding.
Very easy to use, and very flexible.
Let me know if I can help.
The CMS that jeepsunlimited and I use is called Joomla. More information at www.joomla.org.