These source trees come from a Version Control System of some kind.
CVS and Subversion are two popular ones, but the Buildbot supports
others. All VC systems have some notion of an upstream
repository which acts as a server1, from which clients
can obtain source trees according to various parameters. The VC
repository provides source trees of various projects, for different
branches, and from various points in time. The first thing we have to
do is to specify which source tree we want to get.
[1] except Darcs, but since the Buildbot never modifies its local source tree we can ignore the fact that Darcs uses a less centralized model