A role-playing game (or RPG) is a cooperative storytelling game where the players take the role of “characters” that act and interact within a fictional adventure, devised and run by a game moderator (GM).
Since this kind of game is played with friends around a table (or in an online chat or something), it’s often called a tabletop role-playing game to distinguish it from a video game.
The GM is responsible for the consistency of the story through which the players’ characters (PCs) navigate, and for the actions of other characters that the PCs meet in the game.
Each player portrays his or her one character and the GM portrays everyone else. These other characters played by the GM are called non-player characters (NPCs). Obviously, this places a large amount of responsibility on the GM’s shoulders.
A role-playing game by its nature is rather fluid. Unlike other games, it doesn’t involve action on a computer or TV screen, or markings and movements of pieces on a board. It’s much more abstract.
Usually, the GM describes the situation in the adventure to the players, explaining what the PCs see and hear. The players tell the GM what their characters wish to do.
Sometimes interaction between the PCs and the GM is resolved simply by talking; the GM decides something will happen, and it happens. Often, such decisions are made when the outcome of such an event is not in doubt (for example, a character wants to open a door, so the GM decides he does so).
However, when an outcome is in doubt, such as when a PC is attacked by an NPC, or when a PC wishes to complete a difficult task, the GM may ask the player to roll dice and consult the statistics of his or her character. This adds a level of excitement and uncertainty to the game. Character statistics are a numerical value indicating how well a character can do something, whether it’s lifting a weight, hitting someone in the jaw or shooting a gun.
A successful roll indicates success in that particular action, and the GM continues the story accordingly. Many such rolls may be made over the course of a single adventure.
Sometimes the PCs will try to resolve some linear “mission” within an adventure, but sometimes there is no absolute goal. Like real life, the PCs act and react just like normal people, in a world of the GM’s creation. Well-played PCs should be like real people, with their own wants, hopes, dreams and fears. And sometimes there is a short-term goal for them to accomplish, like there is in life: completing a class, delivering the newspapers, or blowing up the enemy stronghold.
Ongoing role-playing games are often composed of dozens of adventures linked together to form a campaign. Campaigns cover months or even years in story time, and follow the paths of the PCs as they grow and develop.
Sometimes, however, an adventure is a one-time thing. The GM prepares and presents an adventure for the PCs that runs its course in a single night, or in a short period of time.
And that, in a nutshell, is role-playing.