(with thanks to Slakr)
Most people understand that games restrict a player’s actions. In soccer you are not permitted to handle the ball, for example, and in Halo you may only carry two weapons at once. The ways by which that a game restricts the player are called the game’s rules.
Some rules penalise or restrict, but others enable. In video games, some rules even allow players to do things that they could not ordinarily do. There are three different types of rule that operate in a game world, and they are constants, constraints, and conditions.