A cheating device or game-altering device is any device that lets somebody do things on a game that may not be possible in normal gameplay, through measures such as editing the memory (e.g. GameShark) or making temporary patches (i.e. Game Genie) to the game.
Game-altering devices are commonly used for finding glitches and unused content. Common cheating devices include GameShark, Action Replay, Game Genie, and Codebreaker.
A memory viewer displays data (typically in hexadecimal bytes) from a number of areas. For Game Boy/Color games, these include the ROM, VRAM, SRAM, RAM, WRAM and I/O. Writable areas such as the WRAM (used for in-game variables) can be edited to achieve things such as forcing Pokémon to appear. For example, editing the memory address D059 in Pokémon Red and Blue is equivalent to using the GameShark code 01xx59D0; the "instant encounter" with a Pokémon or Trainer code.
The GameShark is a very commonly used cheating device, owned by Mad Catz.
GameShark has also become a verb, i.e. "to shark" meaning to use a GameShark or "sharking", which refers to using a cheating device like the GameShark.
The Game Boy GameShark code format is 'ttvvaaaa'. tt is usually 01. vv is the index number of what you want to change an address to. "aaaa" is a two byte value that represents a memory address with the lower byte first.
Example code: 010259D0 (010259D0). This writes 02 to the address D059.
Action Replay is a very commonly used cheating device, created by Datel.
Game Genie is a cheating device originally designed by Codemasters. It is unique for making temporary patches to the ROM rather than changing the memory.
An original physical Game Genie for Game Boy will load an unplayable palette for Pokémon Yellow and Pokémon Gold and Silver/Pokémon Crystal if they are played on a non-DMG Game Boy, however the palette can be reset to a playable one with arbitrary code execution with the Game Genie codes intact. No method however is documented to play Pokémon Crystal on a physical Game Genie, due to the "This Game Pak is designed only for use on the Game Boy Color." message.
Code Breaker is a cheating device made by Pelican. It is not as often used as the GameShark or Action Replay.
Xploder is a cheating device currently owned by Xploder Limited; a company based in England. GameShark codes work perfectly on the Xploder GB.
BBME-IF, standing for "Band Brothers Music Editor-IF" was designed to be a USB based save data reader and writer for Nintendo DS, specifically for the game Daigasso! Band Brothers. The save file editor Pokésav offers support for it. (More information in Japanese).