Game System is a Role-Playing Game term. It refers to the set of Game Mechanics that regulate gameplay and simulate actions that can be performed by the game's characters. The purpose of a game system is to avoid the "Cops and Robbers" problem — when one player can say "Bang, I shot you!" and the other can say "No you didn't, I dodged so you missed!", no meaningful progress can be made. What a Game System does is set down rules for how well the players can shoot and dodge (among other things), so everyone can agree on what actually happened. Given the sheer number of things that a player might attempt in a Role-Playing Game, Game Systems can be correspondingly complex.
Most Game Systems involve using numerical statistics to mathematically chart a character's abilities, though the exact stats and the nature of their use varies widely between systems. In addition, most systems generally involve some randomness, so that there's an element of chance in most actions — you can't be 100% certain beforehand whether your attempt will succeed or fail. In Tabletop Games, this typically takes the form of dice-rolling, while Video Games use random number generators instead. In addition, there's typically one person who takes the role of "referee", controlling NPC's actions and deciding how rules apply in specific situations. The computer AI does this in Video Game RPGs, but it falls to a human Game Master to do this in Tabletop Games — this, unsurprisingly, means that tabletop RPGs tend to be much more flexible and fluid than computer-based ones; a program can only handle what it's been programmed to, while a person is much more capable of improvisation.
Game Systems can be divided into two general categories: Character Class Systems and Point Build Systems. A Character Class System has each player pick their character's "class" (which represents a specific skillset, like combat or stealth) and allows them to advance according to that class's abilities. A Point Build System, on the other hand, allows players to spend character points on whatever Skill Scores and Perks they want; balance is provided by the cost of abilities and how many points players are allowed to spend, rather than balancing specific classes against each other.
There is a lot of overlap between the two types of systems, especially in the from of sub-systems. Game designers want to prevent characters of a given class from being identical, so in order to encourage Character Customization, they frequently build a Point Build System into their Character Class System. This will usually take the form of some "universal" abilities available to all classes and purchased via point buy, having point-buy abilities within a class so that different characters of the same class may have different abilities, or both.
Originally a Game System and its setting were tightly integrated — if you wanted to play in a certain setting, or genre, you bought a book or boxed set that described both the system and the setting, and/or an 'adventure pack' that detailed a pre-made storyline for the players to follow. However, over the years the concepts of House Systems and Universal Systems have emerged. House Systems, as the name implies, are house rules taken to their logical extreme — home-brewed systems consisting entirely (or at least largely) of house rules. Universal Systems, on the other hand, are deliberately designed without a specific setting or storyline — they usually consist of the basic framework (such as dice-rolling and combat mechanics), but also have rules that allow a player to adapt his favorite novel or computer game for use as a setting.
Specific Game Systems sometimes go through dozens of revisions. Each new version can be expected to either clarify existing rules (if ambiguities have arisen in previous versions), add entirely new mechanics to the system (e.g., adding a new section for vehicular combat rules), or streamline existing rules that have been discovered to be unwieldy or poorly-designed.
Due to the nature of its design, a Game System's rules are not all-encompassing — some ambiguity is usually left as a "fudge factor" so that a DM can create new rules to suit a particular situation. Such ambiguity can also allow the player some leeway, by creating loopholes in the game's rules that permit him to do what the rule system would normally prevent. Such loopholes, if found to present an unfair advantage to a player, are usually closed in subsequent revisions.
- The d20 System, originally used by Dungeons & Dragons since 3rd Edition. Available under a free license, it has a huge number of settings and spinoff systems related to it.
- Mutants & Masterminds is a superhero-themed system heavily modified from d20, to the point of being completely Point Buy.
- Pathfinder is the Spiritual Successor to D&D 3.5, created after the introduction of D&D 4E.
- Monte Cook's World of Darkness
- Legend System is a D20 hack, mostly notable for a really neat multiclassing system.
- BEER Engine
- Hero System
- Guardians of Order's Tri-Stat system, used in:
- By White Wolf, we have the Storyteller system, used in:
- Trinity Universe
- The World of Darkness
- Chronicles of Darkness uses the Storytelling system, a cousin system.
- F.A.T.A.L., an infamously horrible "adult" RPG. It uses a... unique game system, which involves 1d10000's, among others, and occasionally can result in characters achieving negative values in attributes, such as anal circumference.
- Forged in the Dark was originally designed for Blades in the Dark, but has since been released as a free SRD and used to run games in entirely different genres (Scum and Villainy, Band of Blades, etc.).
- Fate: Accelerated Edition
- Genesys: the Universal System version of Star Wars: Roleplaying Game.
- Legend of the Five Rings
- R. Talsorian's Interlock system, used in:
- Mekton, a Humongous Mecha game.
- Cyberpunk 2020 (though with a little bit more detailing in the combat).
- The Palladium system, used by Rifts, Palladium Fantasy, and others.
- Powered by the Apocalypse: Originally developed for Apocalypse World, it has since been adapted to a number of other RPGs, e.g. Dungeon World, Monster of the Week, Monsterhearts, Spirit of '77, etc. While not a Universal System per se, it is designed to be lightweight and easily hackable, making it uniquely easy to adapt to any setting and narrative genre.
- Savage Worlds from Pinnacle Entertainment Group.
- Fantasy Flight's Genesys, believed to be a contraction of "Generic System", which started out as the system used for Edge of the Empire et al. Based on rolling dice with symbols on them and interpreting results from there.
- The GYRO (Green, Yellow, Red, Out) System used in Sentinel Comics: The Roleplaying Game, so named for its main combat mechanic where your character has different "zones" that they move through as they take damage with each zone giving you access to more powerful abilities.