What is the framework
From Joomla! Documentation
The Joomla! Framework, renamed the Joomla! Platform is the set of files located in the libraries/joomla folder of your Joomla installation.
Starting in 2011 the Joomla! Platform will also be available as a separate downloadable package.
The platform files contain the set of PHP classes which implement what is collectively known as the Joomla! API.