Many of us have come to understand Joomla! to refer to a specific software we use, yet there are 3 different types of software available from Joomla!
These are:
1) The Joomla! CMS;
2) The Joomla! Platform; and
3) The Joomla! Framework.
What are the differences?
The Joomla! CMS is the product you likely know and use. It's the Content Management System which powers many sites. Usually when talking about Joomla!, we just mean the CMS. Official site: http://www.joomla.org/
The Joomla! Platform is the old underlying code which powers the CMS. It used to be a separated project but was merged back into the CMS. The platform no longer exists as an own project
The Joomla! Framework is a rather new project. It's meant as a framework to power various standalone applications. It's based on the CMS code, but was modernised and improved a lot. Currently, the CMS only uses a few classes from the framework, but that is likely to raise over the time. There are also other applications like JIssues which are built on this framework. Official site: http://framework.joomla.org/
@Bakual did a good job of explaining it, but I just wanted to add that a big purpose of the Framework is to improve practices for everyone who develops extensions for the CMS. It follows modern practices and makes it easier to write clean, testable code. I say this from the standpoint as a big proponent of the Framework (I've contributed heavily) and from the fact that I use the Joomla! CMS every day at work, and I want to make that easier.
The Framework is the future!
