MusicBrainz in a Nutshell

What is MusicBrainz?

MusicBrainz is a user curated database with information about music. It doesn't actually have the music, only information about music. R. Kelly's Chocolate Factory is a good example of the kind of information it (can) store.

Who uses MusicBrainz?

• Your digital music service - Spotify, YouTube, Google Play, Amazon, 7digital, MusixMatch, 8Tracks, and more use MusicBrainz data to improve on the data they get from labels.
• Your radio station - BBC uses MusicBrainz extensively for anything related to music. Several other radio stations also use MusicBrainz data either to improve their internal database or to simply replace it.
• Your fans - may be using a MusicBrainz powered tagger such as Picard or beets to tag music of yours they have bought with additional metadata or play your music using a MusicBrainz aware music player (e.g., foobar2000 or VLC) or media center (e.g., Kodi). A non-exhaustive list of applications that utilise MusicBrainz data to some degree can be seen at MusicBrainz Enabled Applications.
• Your search engine - Google is using MusicBrainz for its "OneBox" when you search for musical artists and WolframAlpha is using MusicBrainz to answer music related questions.
• researchers and scientists - MusicBrainz data is being used for research by students and scientists at Universitat Pompeu Fabra (Barcelona, Spain), Queen Mary University of London (England, UK), and many other institutions around the world.

A comprehensive list of the bigger actors using MusicBrainz data can be found at https://metabrainz.org/supporters but there are also a lot of other smaller web sites and other services in the wild that use the data but which MusicBrainz/MetaBrainz do not necessarily know about.