The PostgreSQL source code is stored and managed using the CVS version control system. An official mirror using Git is also available, for those who wish to use a distributed version control system. This mirror is automatically updated whenever the main repository changes, so it always contains the latest versions of all branches.
Using Git is the most flexible way to work with the source, and it allows you to work offline without having constant access to the project servers. rsync based CVS also lets you work offline, but lacks many of the other advantages of Git.
Our wiki, http://wiki.postgresql.org/wiki/Working_with_CVS and http://wiki.postgresql.org/wiki/Working_with_Git, has additional details on working with CVS and Git.