Originally Posted by Wikipedia
In role-playing games, participants play the parts of characters in an imaginary world that is organized, adjudicated, and sometimes created by a GameMaster (aka narrator, referee, dungeonmaster(DM), storyteller). The gamemaster's role is to provide a world and a cast of characters for the players to interact with, and to adjudicate how these interactions proceed. In addition, the gamemaster may also be responsible for advancing some kind of storyline or plot, albeit one which is subject to the somewhat unpredictable behavior of the players.