Hi all,
Okay, for those totally new to RPGs, the Game Master (aka the Dungeon Master, which is copyright Wizards of the Coast) is the guy or gal that is in charge of everything in the game that isn't the player characters. He (I'll use the male pronoun, but I don't mean to leave out the female GMs, of which there are many good ones) sets the stage that the PCs tell their story on. He is every inn keeper, tax collector, orc, goblin, necromancer and dragon in the game.
He also acts as a judge, settling rules disputes and answering rules questions.
Game Mastering is by far the most challenging and dynamic role to take in an RPG. It takes up the most time and involves the most "work." However, some - including me - consider it the most rewarding aspect of role playing.
This hobby succeeds or fails by the talent and skill of game masters. Being able to juggle the roles of story teller, game designer, judge and character actor isn't for everyone, but if you think it might be for you, read on.
Even if you have no experience at all, you can learn to game master. The first RPG I ever played was one that I ran, and my first experience here on TTW was - you guessed it - running a game. Further, it was a game system (Pathfinder) that I had no experience with at all.