It's a device that lets you essentially edit the game using different codes. By using these codes, you can make yourself invincible, have unlimited items, etc. You become the god of the game. Some people don't like them because they take the challenge out of a game, but you can still do really cool things with them like cheatcodes, texture/character hacks, and sometimes outtakes that weren't included in the final making of the game. Most game systems just need you to enter in codes (which you can find online) then use that code and put in the game. Most stores like walmart, gamestop, or any electronics store should have a gameshark.