Editor, formerly 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 (through a vote) is called an auto-editor (see that page for the extent of those privileges).
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.
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.