The Nintendo GameCube–Game Boy Advance Link cable is a cable used to connect the Nintendo GameCube (GCN) and the Game Boy Advance (GBA). Small games can be downloaded into the GBA, the GBA may be used as an extra screen to supplement gameplay, or the GBA can be a separate controller. By using this cable and a Game Boy Player accessory, a Game Boy Advance can be used to control a Game Boy game being played on the GameCube. The cable has one end that plugs into a GameCube controller slot and another end that plugs into the GBA's extension port. It is the descendant of the Transfer Pak. The cable is compatible with the Nintendo GameCube and Wii on the console side, and the Game Boy Advance, Game Boy Advance SP, Game Boy Player, and e-Reader on the portable side. Because the Game Boy Micro has a differently-shaped link port, the official cable does not work with it, but resourceful enthusiasts have been able to hack together home-made versions which do.
With the advent of wireless technology, this method of communication has been supplanted in newer Nintendo hardware by using the built-in wireless capabilities of the Wii and Nintendo DS.
Source: Wikipedia, "Nintendo GameCube–Game Boy Advance link cable", available under the CC-BY-SA License.