Editor, formerly Moderator, is the main type of user who contributes changes, called edits into the MusicBrainz Database. 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).
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.