An editor, formerly called a moderator, is the main type of user who contributes changes, called edits into the MusicBrainz Database. In the database schema, editors are still internally called moderators through the names of the tables holding the data on editors.
A registered user who has been given additional privileges (after a vote) is called an auto-editor (see that page for the extent of those privileges).
To keep track of the edits an editor performs, other users/editors can subscribe to editors. All edits by editors one has subscribed to can then be reviewed at the subscribed editors page. Those edits, however, are not listed in the daily digest mail you receive for your subscribed artists (see also label subscriptions).
Certain editors (users) are affiliated with other organizations. A prominent example of these are the BBC Teams Editing MusicBrainz (see that page for their usernames). As part of the ongoing collaboration Between MusicBrainz and the BBC, in which BBC is for example using the live data feed to improve the music pages on their websites, employees of the BBC are tasked with improving the MusicBrainz database. They are mainly adding URL relationships to improve the linking between different sources of music metadata. Especially in the beginning of this arrangement, other editors were encouraged to keep track of their edits to the database. This was to account for the fact that 'BBC editors' have to conform both to their employer's wishes and to the MusicBrainz standards and guidelines, and the obvious mismatches that could occur between these two.
Bots are programs that are able to do automated editing of the database. An example of this is the ModBot, which is strongly involved in the daily flow of edits. See ModBot for its duties; most users/editors will come into contact with it through their regular editing.