A system is a book, or set of books, defining a perticular setting or approach to a game. The system will include some means of defining characters by means of stats. and for adjudicating the success or failure of actions attempted by the charcters in the game. In most cases a system also describes a particular environment or setting in which adventures can be set.

There are uncountable numbers of different systems, and more appear every year. These range from the comic to the deadly serious, encompassing fantasy, cyberpunk, realism, science-fiction, horror, espionage, cartoons, comics and more. These systems can be divided into three main classes:

1. Genre Systems
   These are systems designed for a particular style of play, such as heroic fantasy, or space opera. The most famous genre sytems are Dungeons and Dragons and Traveller. There are also many smaller genre systems, like Toon (Cartoons), Floating Vagabond (Comic Space Opera), and After the Bomb (Mutant future Animals).

2. Generic Systems
   Generic systems attempt to provide a basic set of mechanics which can be used in any setting. This set is then generally supplemented by more detailed guides for particular settings. The most famous of these systems is GURPS (Generic Universal Role Play System), which has supplements ranging from James Bond spy Adventures to Marvel Comic book heroes.

3. World Specific Systems
   The largest group fo games are those specific to a particular world, either one invented for the game, or one based on a famous book or film. Thus there are Middle Earth systems, Cthulhu systems and Amber systems. This class also includes games with self contained worlds designed for that games, such as White Wolf's World of Darkness (Vampire, Mage, Werewolf, Wraith and Changeling), or the wargame offshoot Warhammer Fantasy Roleplay (WFRP).

It is important to remember that a system is nothing more than a framework designed to help the players play a specific kind of game. Most groups will tailor (or customise) the system's rules to their own needs, ignoring some rules and adding others. No two games are exactly alike just because they share the same system. However familiarity with a sytem does make it easier to play another game in that system, because the eplayers are familiar with the rules and conventions used in that game. Thus roleplayers will often speak of themseleves as playing a number of specific systems, e.g.

Compare Systemless Games.