A secret (or out-of-boundary) is a 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 in-game, but they can be accessed through 3rd party trainers. The Andromada plane in Grand Theft Auto: San Andreas is a real vehicle used in a cutscene, but is never made available to the player but there is a secret way to obtain it.

GTA III

GTA San Andreas

Easter Eggs

A hidden sign in Grand Theft Auto III

An is an intentional surprise or present placed into the game. For example, in San Andreas, there is a sign on top of the Gant Bridge in San Fierro saying "There are no Easter Eggs up here, go away".

GTA III

• In Staunton Island near Liberty Campus, one of the Pedestrians that can spawn there is a long haired ponytailed college student in a coat carrying a notepad. When killed, the player can zoom in on his notepad, which has a drawing on it depicting a penis either urinating or ejaculating.
• Two computers in the TW@ internet cafe have screenshots from GTA1 and 2 on them each.

GTA IV

• Statue Of Happiness, If you take a helicopter to the statue of happiness and jump out onto a certain platform, you can go through a door and climb up a ladder to see a giant beating heart
• There is an unknown location on the map where you can see graffiti of past GTA artwork on a wall

Grand Theft Auto V

An easter egg in GTA V

See also

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 'GTA Clone' "True Crime: Streets of LA". Usually they are categorized as Easter eggs.

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.

Source: gta.wikia.com