A Release Group is a MusicBrainz concept introduced in May 2009 used to group several different releases into a single logical entity. Every release belongs to one, and only one release group.
Both release groups and releases are "albums" in a general sense, but with a slight difference: a release is something you can buy as media, e.g. a CD, a vinyl record etc. on its own, while a release group embraces the concept of an album -- it doesn't matter how many CDs or editions/versions it had. When an artist tells you "We've released our new album", they're talking about a release group. When his publisher says "This new album gets released next week in Japan and next month in Europe", they're talking about the different releases that belong in the release group that the artist told you about.
MusicBrainz automatically considers every release in the database to be part of a release group, even if this group only contains the one release. As an editor you don't have to worry about creating release groups, you will only need to merge existing ones.
- Transplant's single "Diamonds and Guns" is a single release inside its own group.
- Weezer's "Weezer (Red Album)" has ten editions in the database, some releases from different countries, some deluxe editions, and one transliteration.
- Franz Ferdinand's "Tonight: Franz Ferdinand" was issued on its own, with a bonus disc, and on two vinyl records.
- A 3 disc self-titled compilation by "Nirvana".
- Mozart's 9 volume, 170 disc "Wolfgang Amadeus Mozart: Complete Works" box set, or Enya's 3 disc box set "A Box of Dreams".
- Blind Guardian's "Nightfall in Middle-Earth" was first released in 1998 and then remastered with a bonus track in 2007
- The musical "My Fair Lady" (Original London Cast) was originally released on vinyl in 1959 and on CD in 1998.
See the dedicated page in the style section.