CMFDefault ships as part of the CMF product. It was initially intended as a demo implementation of a content management system. In practice, some parts of it are often used by applications which are developed on top of CMFCore and CMFDefault. For example, CPS and Plone take this approach.
Out of the box, CMFDefault might be described as rather plain. Many people prefer to start with a more feature-rich and polished application such as CPS or Plone. CMFDefault is simpler, and so might be a good choice as your starting point if your goal differs radically from the sort of community-portal sites typically implemented with Plone and CPS. This approach can be simpler, but can also lead to reinventing the wheel.
CMFDefault provides several ContentTypes and several CMFTools, as well as some glue code to integrate features such as DCWorkflow.
The ContentTypes all implement DublinCore metadata. They include Document, Image, News Item, File.
The tools include DiscussionTool?, MetadataTool?, RegistrationTool?, URLTool?, MembershipTool?, PropertiesTool?, SyndicationTool?.
Other tools are also provided by CMFCore.