Its a very customizable Content Management System (CMS). I prefer drupal more, but Joomla is pretty good.
Joomla is a way to handle content, whether it be in text form for articles or content in other forms, such as video or audio. There are not many other forms of content online so far. When you hear "Content Management" just think content handling. Joomla is "Content handling system" ( CHS ) but CMS is more accurate. Joomla handles content very well and intelligently. Here is a link to see exactly how Joomla defines itself : LINK. And here is where to start, if you want : LINK Good luck.
With CMS (Content Management System) you can easily manage your article/content in your site and Joomla is one of great CMS
It is also very useful if your designing web sites for clients. Clients can easily change content and update as they go...not much work for the webmaster after the initial setup
What is Joomla? Joomla is an award-winning content management system (CMS), which enables you to build Web sites and powerful online applications. Many aspects, including its ease-of-use and extensibility, have made Joomla the most popular Web site software available. Best of all, Joomla is an open source solution that is freely available to everyone. http://www.joomla.org/about-joomla.html
Joomla is one of CMS (content management system) where you can build new websites easily by integrating other components depends on your need. CMS itself is a system that makes you easy to add contents, menu and links to your websites without have to recode the HTML. CMS has administrator/backend part where you can administer your websites like adding content, create menu, assign link, manage users, manage images and many more. Joomla is powerful because it has many nice and beautiful templates and powerful components as well. You can add for example video component to host video like youtube, or add ecommerce component to sell your products and many more. It's the best CMS for me.
It's basically a web platform that speeds up the site building process, skipping many steps. It's a breeze really, and highly recommended over hand coded websites.