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 ries.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.