All games are bound by rules. Rules restrict or empower player actions, and so define the game world as different from the physical world. Most rules come from inside the game, but some are external impositions into the game. Rules are a type of lever (the other two types being resources and actions) that make up the frame of the game.
Rules can be tangibe or intangible. A tangible rule is one about which the player has direct knowledge, such as the damage value of a sword, the cost of an item or the exact sequence of button presses required to activate a special move. An intangible rule is one that the player can feel the effects of, but does not know its exact variables. Gravity or character behaviours are usually governed by intangible rules. Videogames are more like sports than board games in that most of their rules are intangible.
There are four types of rule are: Creative Constants, Conventional Rules, Constraining Rules and Conditional Rules.