The Dungeon Master lays out your quest and keeps the game alive. Usually has been playing Dungeons and Dragons for more than half a year or so.
WoW(World of Warcraft)
Probably the first instance any alliance player will go through. Contains iffy and somewhat decent gear for players who are around levels 12-30.
Person 1: darn, I really wish we had a DM.
Mysterious person: I...will be your Dungeon Master.
Person 1: hey guys, lets do DM.
Person 2: kool, ive been waiting for that!