From NPC Essentials Introduction:
Game mastering is a demanding job. If you stop to think about all the tasks a Game Master must perform, it is truly staggering. He must play the role of every NPC, monster, or other “non” player element in the game. The Game Master must prepare adventures and maintain an over-arching campaign. She must balance game play so that it is challenging, entertaining, and most of all, fun. Considering the amount of effort that must be put forth, it is a tribute to the role-playing game community that so many players eagerly don the mantle of Game Master!
Game mastering is a craft, one that takes years to perfect. The GM Mastery series is designed to accelerate this learning curve. It is the goal of this series to give Game Masters a toolkit of tips and techniques to lesson the burden of running games and make the experience richer for players.