The set of rules of a role-playing game is known as its game system; the rules themselves are known as game mechanics. Although there are game systems which are shared by many games, for example the d20 system, many games have their own, custom rules system.
Many role-playing games require the participation of a gamemaster (GM), who creates a setting for the game session, portrays most of its inhabitants, known as non-player characters (NPCs) and acts as the moderator and rules arbitrator for the players. The rest of the participants create and play inhabitants of the game setting, known as player characters (PCs). The player characters collectively are known as a "party".
During a typical game session, the GM will introduce a goal for the players to achieve through the actions of their characters. Frequently, this involves interacting with non-player characters, other denizens of the game world, which are played by the GM. Many game sessions contain moments of puzzle solving, negotiation, chases, and combat. The goal may be made clear to the players at the outset, or may become clear to them during the course of a game.
Some games, such as Polaris and Primetime Adventures, have distributed the authority of the GM to different players and to different degrees. This technique is often used to ensure that all players are involved in producing a situation that is interesting and that conflicts of interest suffered by the GM are avoided on a systemic level.
Games rules determine the success or failure of a character's actions. Many game systems use weighted statistics and dice rolls or other random elements. In most systems, the GM uses the rules to determine a target number though often the targets are determined in a more principled fashion. The player rolls dice, trying to get a result either more than or less than the target number, depending on the game system. Not all games determine successes randomly, however; an early and popular game without random elements is Amber Diceless Roleplaying Game by Erick Wujcik (1990).
Most systems are tied to the setting of the game they feature in. However, some universal role-playing game systems can be adapted to any genre. The first game in 1981 to feature such a system, Champions, is accompanied by a number of sourcebooks which allow games to be created in different genres. The d20 system, based on the older role-playing game Dungeons & Dragons, is used in many modern games such as Spycraft and the Star Wars Roleplaying Game.
In practice, even universal systems are often biased toward a specific style or genre and adaptable to others. For example, although the d20 system has sourcebooks for modern and futuristic settings, most published d20 system material stays within Dungeons & Dragons' combat-focused fantasy milieu.
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