This page documents Secrets, Easter Eggs and Myths in the GTA games.
An Easter Egg is an intentional surprise or present put into the game, found only by those who play the game a lot. For example, there is a sign on top of the Gant Bridge saying "There are no Easter Eggs up here, Go away".
A Secret is an unknown part of the game that is usually not intended to be accessed. For example, the buildings where cutscenes are animated often aren't normally accessible ingame, but they can be accessed through 3rd party trainers. The Andromada plane in GTA San Andreas is a real vehicle used in a cutscene, but is never made available to the player, but it can be spawned then entered and flown like a normal aircraft.
GTA games also contain a large number of References to popular culture, usually in their advertising, such as billboards showing "True Grime: Street Cleaners" as a reference to the popular "True Crime: Streets of LA".
Cheats are manual codes entered to alter gameplay, such as. These are created intentionally by Rockstar to allow gamers to have occasional fun without having to spend hours searching for the right vehicle or escaping a wanted level. However, many gamers use cheats throughout gameplay.
Modifications are alterations to the games such as vehicles, buildings, maps, player skin, textures, missions etc created by third-party sources (not Rockstar). Also, many tools exist to alter the gameplay experience, called Trainers, which can modify any number of things in real-time whilst you're playing a game, from spawning cars to enabling flight.