From Bulbapedia, the community-driven Pokémon encyclopedia.
The GameShark is a cheating device used to alter the internal data of many video games. It is well-known for its use in the Pokémon series, as it is one of the ways to easily obtain legendary Pokémon or Pokémon only available through an event, and many rare items, such as Master Balls, Rare Candies, or vitamins. Also, if too many codes are entered in a GameShark, it may stop working.
Use of a GameShark can sometimes cause corruption of a save file. This is especially likely if the game is not inserted correctly into the slot or if it is bumped while playing. In the Generation I games, complete deletion of the save may occur if the game is taken out while the GameShark is running.
A Game Boy Color GameShark
A GameShark code is made up of three parts. The first part is the code type, which is nearly always 01. The second is the quality digits, which are the digits that will go into that space in memory. This is usually represented by XX or ??, as these numbers can be modified to change what will be received. All values are in hexadecimal, instead of decimal (for example, 31 in decimal is represented by 1F in hexadecimal). The third and final part is the address in memory, represented in little-endian notation. For example, if the data at address C056 is to be modified, the end of the code would need to be 56C0. The code 010356C0 would tell the GameShark that the code type is 1, and that the value 03 will be written to the address C056.
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Reason: information on Generation III and Generation IV.
If a code is incorrectly entered, the game may give the player a Bad Egg, rewrite the Bag contents, freeze the game, corrupt or delete a Pokémon, corrupt the Hall of Fame data, or simply corrupt the save file. These occurences may also occur at other times. In other situations, nothing unintended may occur.