Source + Dummy: This is a skeleton site and the TYPO3 source itself. Download this if you’re going to build a new site.
Source: This is the source of TYPO3. Download it if you have a running TYPO3 installation and would like to upgrade to a new version.
Dummy: This is just a skeleton site. Download it if you already have the TYPO3 source and just want to set up a new site.