In this wiki, we follow the convention of Rulebook 1.0, and always refer to the GM in feminine form and to the rest of the players in masculine form. This will allow some rules to be clearer and more easily understood.
The GM is a storyteller; she is the person in charge of everything other than the player characters themselves. She creates a story for them to play or follows a story created for the setting. The GM sets the goals for the characters and she is in charge of creating drama around the events that will lead the story onwards.
The GM controls all the non-player characters, sets their personalities and interests, decides their actions, and says what they say, even imitating voices (or not). She describes the scenarios where the players find themselves, she makes up how they can interact with them, etc.
It is also her task to determine how the actions of the players impact the story. Additionally, she is the arbiter of all rules. If the group cannot find a solution for a problem the GM always has the final word. She should try to use this responsibility for the better of the story, preventing players from getting caught up discussing rules.
In the end, it all comes down to creating an interesting story that everyone can enjoy, and to have fun together.