The Nintendo 64 Expansion Pak increased the system's random access memory from 4 megabytes to 8 megabytes. It was needed to play certain games that had high memory requirements and sometimes could be used to access optional features.
The Nintendo 64 was one of the last consoles to use cartridge media for games, and these cartridges usually came equipped with an internal battery when needed for saving game data. The system thus did not have memory cards for saving game progress as disc-based systems usually do. The closest thing to an official "memory card" that the Nintendo 64 had was the Expansion Pak, which increased available system memory rather than providing storage space.
The Expansion Pak is plugged into a port near the top of the console. Certain games, such as "The Legend of Zelda: Majora's Mask" and "Donkey Kong 64," will not run at all without the Expansion Pak present. A wide range of games have optional features or improved graphics enabled when the Pak is present, such as "Duke Nukem: Zero Hour," "Gauntlet Legends," "Hydro Thunder," "Perfect Dark," "Resident Evil 2" and "San Francisco Rush 2049."
The Transfer Pak does have memory available for data storage, but it is limited exclusively to transferring data between a handful of Game Boy and Nintendo 64 games that are compatible.