Joomla is part of the very crowded and competitive CMS systems currently available on to download and install. With hundreds, if not thousands of differing systems, processes and functionality, selecting the correct system for your needs can be complex and time consuming.
Some systems are dedicated e-commerce systems allowing you to sell products online, others specialise in online dorectories and so on. Whilst each of these systems have their merits and I would recommend them if the brief required an e-commerce system only for instance.
Joomla on the other hand is a framework that can be expanded as required to fit the needs of your website. Through the thousands of extension components, plugins and modules a myriad of different websites can be developed. Add to this the powerful templating features of Joomla giving developers exact control of how the website will look and Joomla becomes a powerful CMS.
From a simple website with standard content / picture pages, to an online e-commerce site that has a community forum, secure document downloads or even a property database, Joomla can handle all of this, everythign inbetween and more!