Mambo is free software distributed under the GNU General Public License (GPL), Version 2. The full text of the license can be found here: http://www.opensource.org/licenses/gpl-license.php
We won't be going into any detail here about just what the license means as there are many fine resources on the Net that explain the GNU/GPL and this complex issue is just too much for a short tutorial. In essence, the license gives you many rights to use, modify and redistribute the Mambo source code. It does, however, have some limitations on what you can do with it. If you plan to use any of the Mambo code in a modified release, please make sure you read and understand the license.
The name, "Mambo" and the Mambo logos and images are copyright and trademarked by the Mambo Foundation. You can find the trademark and logo usage guidelines on the Mambo Foundation web site at http://mambo-foundation.org
So, you know you can download and use Mambo free of any charge. You can modify it and add to it, you can create your own modules and components for it. And because Mambo is also open source, you can dig around in the code to see how it all works. Open Source essentially means that all the code for Mambo is able to be viewed and edited. You can modify it, hack it, extend it, integrate other scripts into it, and generally do whatever you like with it for your own use.
If you wish to share your modifications etc with others, or redistribute code based on Mambo, the terms of the GNU General Public License and our trademarks must be respected.