From Joomla! Documentation
The Joomla Framework 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.
The Framework comprises code originally developed for the Joomla Content Management System and is a continuation of the Platform project. The Joomla Framework and the Joomla CMS are separate projects with separate development teams. Unlike the Platform, the Framework is not primarily intended as a platform for the Joomla CMS, and the Joomla CMS is not currently built on the Framework. More information about the differences between the Platform and the Framework can be found in the March 2013 edition of the Joomla Community Magazine.