From Bulbapedia, the community-driven Pokémon encyclopedia.
The Transfer Pak (Japanese: 64GBパック 64GB Pack) is a peripheral for the Nintendo 64 that allows for the transfer of data between the console and Game Boy or Game Boy Color cartridges. The device came as a pack-in with the first Pokémon Stadium title, although it could also be purchased separately. It is plugged into the back of the Nintendo 64 controller. With the Transfer Pak, Pokémon and items from Game Boy games can be uploaded into the Stadium games for storage at the Laboratory, or to be used in battles and mini-games. It also allows trading Pokémon (two Transfer Paks required) and allows the playing of Game Boy Pokémon games on a TV using the Game Boy Tower feature.
Data for Pokémon Crystal is contained in Pokémon Stadium 2, but can only be unlocked when connecting a Pokémon Crystal cartridge via the Transfer Pak.
Pokémon games compatible with the Transfer Pak