metapackages – contain no files, are groupings of either individual packages or other metapackages that you create in packageMaker. (example: Mac OS X Install disk is a metapackage; or the iLife installer which is a metapackage is made up of packages for iTunes, iMovie, iDVD, etc)
Distribution Project – single documents (usually within an Installer package) that describe the entire installation experience of a particular product. A distribution script contains all of the resources and logic necessary to provide the complete presentation of the Installers user interface to the user.
The distribution project is used for a complicated metapackage & subpackage project, and is used by the Installer to provide a consistent user experience, even if you make changes to subpackages contained within your metapackage. This is because the distribution project contains all the resources for the user experience: background pics, Read Me text, licenses, etc).