The MusicBrainz Database is built on the PostgreSQL relational database engine and contains all of MusicBrainz' music metadata. This data includes information about artists, release groups, releases, recordings, works, and labels, as well as the many relationships between them. The database also contains a full history of all the changes that the MusicBrainz community has made to the data.
Core data
Artists
Name, sort name, IPI, aliases, type, begin and end dates, disambiguation comment, MBID
Release Groups
Title, artist credit, type, disambiguation comment, MBID
Releases
Title, artist credit, type, status, language, date, country, label, catalog number, barcode, medium(s), disc ID(s), ASIN, disambiguation comment, MBID
Mediums
Format, list of tracks (title, artist credit, duration)
Recordings
Title, artist credit, duration, ISRC, PUIDs, relationships, disambiguation comment, MBID
Works
Title, ISWC, relationships, disambiguation comment, MBID
Labels
Name, sort name, aliases, country, type, code, begin and end dates, disambiguation comment, MBID
Relationships & URLs
Relationships are a way to link the above entities together and allow MusicBrainz to capture most of the data contained in the liner notes of a CD.
CD Stubs
Title, artist, barcode, disc ID, disambiguation comment

Supplementary data includes:

user submitted annotations, tags and ratings
derived statistics
search indexes
edit history
non-personal user data