MusicBrainz is a project that aims to create an open data music database that is similar to the freedb project. MusicBrainz was founded in response to the restrictions placed on the Compact Disc Database (CDDB), a database for software applications to look up audio CD (compact disc) information on the Internet. MusicBrainz has expanded its goals to reach beyond a compact disc metadata (this is information about the performers, artists, songwriters, etc.) storehouse to become a structured open online database for music. 

MusicBrainz captures information about artists, their recorded works, and the relationships between them. Recorded works entries capture at a minimum the album title, track titles, and the length of each track. These entries are maintained by volunteer editors who follow community written . Recorded works can also store information about the release date and country, the CD ID, cover art, acoustic fingerprint, free-form annotation text and other metadata. , MusicBrainz contained information about roughly 1.1 million artists, 1.6 million releases, and 16 million recordings. End-users can use software that communicates with MusicBrainz to add metadata tags to their digital media files, such as MP3, Ogg Vorbis or AAC. 

MusicBrainz allows contributors to upload cover art images of releases to the database; these images are hosted by Cover Art Archive (CAA), a joint project between Internet Archive and MusicBrainz started in 2012. Internet Archive provides the bandwidth, storage and legal protection for hosting the images, while MusicBrainz stores metadata and provides public access through the web and via an API for third parties to use. As with other contributions, the MusicBrainz community is in charge for maintaining and reviewing the data. Cover art is also provided for items on sale at Amazon.com and some other online resources, but CAA is now preferred because it gives the community more control and flexibility for managing the images.
What is CDDB?
the Compact Disc Database