A Gamemaster or Game Master (often abbreviated as GM) is a player in a multiplayer game who acts as organizer, arbitrator, and officiant in rules situations.
Today, gamemaster is usually associated with role-playing games. In a
role-playing game the Gamemaster's purpose is to weave the other
participants' player-character stories together, control the non-player
aspects of the game, and create environments in which the players can
interact. The basic roles of Gamemasters - rules help, moderation, and
storytelling - are the same in almost all role-playing games, although
differing rulesets make the specific duties of the GM unique to that
system.
See the links to the left for games that I am or was the Game Master for.