Dungeon Master, or DM, is a title used for the game master of a Dungeons & Dragons play session.
What does the DM do?: The Dungeon Master has many hats to wear in the course of a game session. The DM is the rules moderator, the narrator, a player of many different characters, and the primary creator of the game’s world, the campaign, and the adventure.
Who should be the DM?: Who should be the Dungeon Master for your gaming group? Whoever wants to be! The person who has the most drive to pull a group together and start up a game often ends up being the DM by default, but that doesn't have to be the case.
Dungeon Masters Can Partner, Trade Off, or Change: The role of Dungeon Master doesn't have to be a singular, ongoing, campaign-long appointment. Many successful gaming groups switch DMs from time to time. Either they take turns running campaigns, switching DM duty every few months, or they take turns running adventures and switch every few weeks.