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.