A Role Playing Game is a game played by a group of friends who step into the shoes of customized heroes living in a mythical and dangerous world (often called a "Campaign Setting", of which Tormentosa is an example). These players, the characters, undertake to collaboratively invent an imaginative history: exploring the wilderness, battling dreadful beasts and uncovering ancient treasures, all the while weaving plots of political intrigue and endeavoring to get filthy rich and inhumanly powerful.
Players usually do all the above using "character sheets" to keep track of their characters, and roll dice to decide the outcome of actions. The game is "referee'ed" by the Master of Ceremonies or "Game Master", sometimes called a Dungeon Maestro, who designs the world, it's multitude of oft-at-odds factions and the interplay of interesting characters.
It is the duty of the MC to enable the players to act out their fantasies within the framework of the ruleset of the game.