The Game Link Cable is a cable used to directly connect two Game Boy or Game Boy Color systems for various multiplayer gameplay. For the core series games, it allows players to battle and trade with other players. The Game Boy Advance has its own Link Cable, and cannot utilize the Game Boy or Game Boy Color ones.
Game Link Cables in general became obsolete after the Nintendo DS released, as the system allowed for players to battle, trade, or chat via wireless means. Even Pokémon Emerald Version, which was released during the Nintendo DS's early years, used a Wireless Adapter for Game Boy Advance units to communicate.