Portal:Platform/Intro
From Joomla! Documentation
The Joomla Platform is a platform for writing Web and command line applications in PHP. It is free and open source software, distributed under the GNU General Public License version 2 or later.
Since 2011, the Joomla! project was split up and the Platform is now being developed as a separate project. The Joomla Content Management System (CMS) is now built on top of the Joomla Platform.
You can download the platform at https://github.com/joomla/joomla-platform
