A GameShark is a standard console cheat device. Unlike a PC trainer, it's usually a physical device that plugs into the cartridge slot and in turn accepts the game cartridge, much like the Trope Codifier, the Game Genie. The Trope Namer comes from the next generation, designed to work with disc-based consoles.
Video game companies didn't like this for two reasons. First, it's cheating. It usually matters where there's some form of competitive gaming scene; if you were aiming for a world record, you had to prove that you did it without cheating. Old video game magazines in particular, when holding contests, would require entrants to send in a picture that showed both the TV screen — with the score on it — and the console itself, to show that it didn't have this kind of device in it.
Second, it could be harmful to the console itself. A well-made product like the Game Genie wouldn't be too bad, but a less robust equivalent could indeed damage delicate internal components.
Third, it could be harmful to your save file. Some games interact with the console in interesting ways and can be made to behave unpredictably with a GameShark, and if the save file is corrupted, bad things can happen — for instance, Animal Crossing on the Nintendo DS will not load at all, because it calls your town at startup for the intro. This is why GameShark users prefer to make backup saves, in case they inadvertently make the game Unwinnable. Later generation devices, like the GameShark proper, Action Replay MAX, and Code Breaker, advertised themselves as not messing directly with your save file and thus safer.
All this being said, courts have tended to side with the device maker on this, on the grounds that if you bought the console and the game, you have every right to decide how you want to play it — including through the use of an external device.
Devices like these are particularly useful for accessing Dummied Out content. Whether accessing this content counts as "cheating" is a matter of debate:
- Most Pokémon games have at least one Mon that you can only obtain legitimately through a company-sponsored event. These are rare and not accessible to all gamers, but the only other way to get these Mons is through a glitch or a GameShark.
- The infamous Hot Coffee Minigame in Grand Theft Auto: San Andreas was Dummied Out after considerable controversy and can only be accessed this way.
- This is the only way to access the almost-finished "Bottle's Revenge" two-player mode in Banjo-Tooie.
- Final Fantasy Tactics has a half-finished two-player mode which can be activated by means of a GameShark — each player has to use the same controller in turn.
- Many Fighting Games have extra unfinished characters or computer-only characters that are playable in this manner.
Some games are infamous for being "immune" to these devices, or at least difficult to play with them without locking up the game. For instance, tri-Ace games like Star Ocean: The Second Story or Valkyrie Profile have non-static coding values and are thus harder to mess with (although you can get around this if you know the "activator code" — which in Star Ocean's case was about 29 lines long).
These cheat devices aren't really in vogue anymore and haven't been since the Sixth Generation, owing mostly to the paradigm shifts in console gaming to online play on one hand and in homebrew scenes to software-based hacks that don't require external devices to install on the other hand. The only company left making these things is Datel, makers of the Action Replay line. You can still get an Action Replay for the PSP, Nintendo DS, Nintendo 3DSnote and Wii, but they can be rendered useless by a single software update — which is why there isn't an Action Replay for the Xbox 360 or PS3 onward. The Action Replays for the PSP, 3DS and Wii have been superseded by homebrew cheat applications while the PlayStation Vita got a homebrew cheat software but never an external cheat device in its lifetime.