The Game Genie
was a device shaped like an NES
cartridge cut in half that when used allowed the player to enter "codes" which modified the game data allowing the player to cheat. It was attached to the end of an NES game and the whole assembly was then inserted into the NES. The codes had many effects, most often giving the player some form of invulernability or infinite ammo or some such modification. The codes came in a little booklet shipped with the Game Genie, but now all of these and many more discovered or created by players can be found online. There are whole websites dedicated to just Game Genie codes.