Most RPGs have a moderator. Known by many names in the business---Game Master, Dungeon Master, Story Teller, and others---it is the moderator who serves as guide to the setting, and arbitrates rules disputes.
It is his job to present the setting, play the roles the players do not---at least those that can be said to have a personality---and keep play running smoothly.
It is he who is the director, the producer, the set designer, the casting director, the props man, the foley man the lighting director, and the stage manager. At times he can be the scenario writer, where he's not adapting the work of another.
Now most people in the hobby call him "master" because the see him as being in charge of the game. This has a tendency to get people to see game play as a competition between player and "master" when that's really not the case. When there is competition between players and outside events, it's usually between the group and a situation, or the group and other groups (internal competition is another matter).
One game personality prefers the term, "Game Moderator" because what the fellow does is moderate the game. I shortened it to "moderator" because it saves on ink/bandwidth.
There are a lot of thing the moderator does, or can do. There are a few things he cannot do.
He cannot insure play balance. He is presenting a world that is just too vast usually, too changable for that to be possible. Even if the game is designed to give the appearance of balance events in the game will rapidly put an end to it. (And don't forget the actions of the players.)
He cannot insure party balance. You cannot make people take part to any degree they don't feel comfortable with. Nor can he make people share, that is a job for the players. No matter what the rules say, or the moderator does, the aggressive player will always dominate the game; unless the other players take measures to assert themselves and rein in the bucking stallion.
Finally, a moderator cannot insure that everyone has fun. That depends on how much the players get into the game.
The last thing to consider here are the so-called "GM-less" games. Here the rules are written so the party as a whole can moderate events, much as a moderator does. They are based on the premise that the players will all want to take part in moderating, which is not always the case.
In some groups the players will pretty much share in running the game. In other groups one player will find himself becoming the effective moderator for the session. It's an idea that can work if the players put some work into it, but it's not going to work out for everybody.
And that's our look at game moderators. Coming next, a look at the players and their characters.