The Transfer Pak (64GBパック Rokujūyon Jī Bī Pakku, 64 Game Boy Pack) is an accessory that plugged into the controller and allowed the Nintendo 64 to transfer data between Game Boy or Game Boy Color games and N64 games. The Transfer Pak has a Game Boy Color slot and a part that fits onto the expansion port of the N64 controller. It is considered the successor to the Super Game Boy, and the predecessor of the GameCube-Game Boy Advance cable. It was included with the game Pokémon Stadium, as the game's main feature was importing Pokémon teams from Game Boy titles.
Pokémon Stadium and Pokémon Stadium 2 are games that rely heavily on the Transfer Pak. Pokémon Stadium also included a "GB Tower" mode for playing Pokémon Red, Blue, and Yellow on the N64 via a built-in Game Boy emulator (which included unlockable "Doduo" and "Dodrio" modes which would speed up the game by a factor of 2 and 3, respectively). The Stadium games are the exception, as normally it is not possible to actually play Game Boy games on the N64 with the Transfer Pak, as was possible with the Super Game Boy on the Super Nintendo Entertainment System.
Both Mario Golf and Mario Tennis also made use of it. Rare's Perfect Dark was initially going to be compatible with the Transfer Pak in order to use pictures taken with the Game Boy Camera for creating characters with real life faces, but this function was scrapped after the attacks at Columbine High School and a wave of anti-violent video game sentiment, and the Transfer Pak was usable only in combination with the Game Boy Color version of Perfect Dark for unlocking bonuses.
Source: Wikipedia, "Nintendo 64 accessories", available under the CC-BY-SA License.