A rules set strives to be complete, but no set of rules covers every circumstance. Rules that are fan-driven and implemented unofficially are termed house rules. Contrast with optional-rules, which are officially created additional rules.
House rules can completely replace certain parts of the rules set, or merely add to the existing rules to make them more complex, complete, and/or well-rounded. Many times they are created in response to a need that the designers overlooked. But just as often, they are created to take the game in a direction fitting to a certain campaign.
This tag should be used to discuss implementation, effects, and/or changes engendered by the use of house rules in the basic rules set.