A content management system is software that keeps track of every piece of content on your Web site, much like your local public library keeps track of books and stores them. Content can be simple text, photos, music, video, documents, or just about anything you can think of. A major advantage of using a CMS is that it requires almost no technical skill or knowledge to manage. Since the CMS manages all your content, you don't have to.
Joomla is used all over the world to power Web sites of all shapes and sizes. For example:
Here are just a few examples of Web sites that use Joomla:
More examples of companies using Joomla can be found in the Joomla Community Site Showcase.
Except taken from Joomla: http://www.joomla.org/about-joomla.html
Before selecting someone to handle the development of your site, it couldn't hurt to check out their site and ones they have developed. Be sure to ask what is included in the price.