Released in September 2002, the e-Reader is an unusual device from Nintendo that's designed to work with the Game Boy Advance. One end of the apparatus plugs in to the cartridge slot of the handheld and the other end holds a thin slit, just wide enough for the edge of a collectable card. When special e-Cards are passed through this slit, in a manner similar to scanning the magnetic strip on a ATM or credit card, the e-Reader detects printed patterns of tiny dots.
The patterns of dots printed on the e-Cards are translated into programming code and other information that the GBA can use. Scanning some e-Cards may unlock new features in a contemporary release, while other cards may hold the code for classic games originally released on the NES. In many cases, several e-Cards hold different pieces to a bigger program, and it may be necessary to collect a certain group of cards in order to get the code for a complete game.
The design of the e-Reader allows a link cable hook-up to another GBA, even when the device is in use. It also allows a connection to the GameCube console, through the GameCube - GBA Cable. In addition to the many e-Cards that unlock auxiliary GBA content or emulate classic games on the handheld, others are designed to add special features to selected GameCube games, for those with all the necessary Nintendo equipment. ~ T.J. Deci, All Game Guide