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.
There are a number of situations in which merging releases groups is appropriate. All of the following can be part of a single release group:
When Not to Group Releases together
There are a number of cases where it is not appropriate for releases to be part of the same group:
Use of Attributes
The title of a release group should usually be the title of its individual releases, removing release-specific Extra Title Information added due to Disc Number Style. In the case of re-releases with different titles, the principle of Consistent Original Data should be applied to choose the release group's title from one of the linked releases.
The types of a release group are the same as those possible for Release Type and should normally be the same as all of the releases in that group. This may not always be the case. It is OK for a bonus disc EP, remix, live disc etc to be inside the same "Album" release group as the accompanying album disc.