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/