The projectb database stores all the metadata about the packages in the Debian archive. It contains what versions of what packages are in what suites for what architectures; and much much more.
The ProjectB database is accessible by DebianDevelopers from the machine mirror.ftp-master.debian.org. Just run the following command there to get into the database using the PostgreSQL interactive client:
psql service=projectb
It is used by DebianDak.