Tabletop role-playing games can be mysterious to those who have not tried them. Their immense variability (genres, characters, scenarios, settings, etc.) can make examples misleading, masking the underlying concepts.
A functional/procedural description:
1. The gamemaster describes your character's perception of the present situation.
2. You tell the gamemaster what, if anything, your character does or tries to do.
3. The gamemaster determines the results of your character's efforts.
In step 1, the gamemaster also divulges any relevant information that the player lacks but which his character is presumed to know or believe as a result of his background and experiences, which are assumed to be as rich and detailed as a real person's. In addition, players are entitled to ask the gamemaster for elaboration based on what the character is able to observe; for example, if the gamemaster mentions a horse that your character sees, you can ask for its colour, size, etc.
For the purposes of step 2, not doing anything (e.g., standing still) is considered an action. Furthermore, what counts as an action varies with the current time-scale. During combat, for instance, where the time scale is typically moment-by-moment, actions—such as striking a blow, drawing a weapon, or moving to another location—are correspondingly brief and immediate. On the other hand, the time scale for cross-country travel might be hourly, daily, or longer, depending on the distance to be covered and the level of detail the gamemaster wants to bring into play; an uneventful, routine journey can be narrated in a few words.
In step 3, the gamemaster determines whether your character's action succeeds. Dice (or other randomizers) are used to resolve uncertainty, such as whether an arrow finds its target. Routine endeavours are typically assumed to succeed. Nevertheless, all player-character (PC) actions, no matter how trivial-seeming, are subject to gamemaster approval. The gamemaster then incorporates the results of your character's action into the new status quo, which is then described in step 1 as the cycle is repeated, ad infinitum.
A role-playing game, then, is an endlessly iterative process. Characters are immersed in and interact with an imaginary world, constantly bringing about a new state of affairs. (Fans of German philosophy might recognize a pattern of thesis-antithesis-synthesis.)
The gamemaster, who alone decides and fully knows what is true inside the game universe, represents himself to the players as their characters' senses and sometimes their memories. In this role, he is supreme master of the in-game (fictional) reality. To the players he is like Plato's cave master, Descartes' evil daemon, or the simulated reality in The Matrix.
A crucial distinction between role-playing games and conventional storytelling is that the gamemaster does not control the protagonists; the players do. While the player controls only the will of his character (virtually everything else is managed by the gamemaster), his role is anything but insignificant, for the game is always centred on player-directed character exploits.
Yet a player never has total freedom of action. For instance, his character must contend with obstacles and barriers within the game world, such as antagonists, monsters, or difficult terrain. In addition, the gamemaster can try weaving plotlines into the characters' lives, but players seldom do what is expected, tempting the gamemaster to introduce a deus ex machina. Not surprisingly, the excessive manipulation of events by the gamemaster is frowned upon. Termed railroading, it transforms players into mere spectators, undermining the essence of the game.
Nonetheless, the gamemaster will sometimes override a player's control of his character—either to simulate the loss of free will, such as when succumbing to a Siren's song; to prevent a player character from acting on knowledge that he doesn't have (meta-gaming); to compel behaviour to which the player had already committed himself; or to retire a character from active play, effectively converting his status to that of non-player character (i.e., a character under gamemaster direction).
Being a game, it is natural to wonder how one wins. But role-playing games aren't about winning; their open-ended structure precludes ultimate victory conditions. The goal of role-playing games is for players to have fun, to experience the thrills of danger that are as real as imagination permits. Gamemasters try to make the fictional environment compelling and engaging. Character motivations, on the other hand, are for the player to choose. Whether he succeeds or fails, the character's life—and the game—goes on.
Players don't lose even if their characters die; in adventure games, the threat of death must be credible to maintain excitement. Thus, characters are occasionally lost, often heroically or tragically. But unlike in real life (as of this writing), the player simply creates another character and rejoins the game. And, until real life catches up, let the game continue!
What strikes me about this is the prime important of the 'gamemaster.' In contrast, both of the descriptions of larp barely mentioned the gamemaster. This isn't just the role of the TT GM in presenting the setting (which is a natural consequence of the format), it's the position of authority/agency/narrative control.