A Memory Card is a video game accessory that has been a part of many gaming consoles. Essentially it is an external memory database where game data is saved.
The Nintendo 64 was the first Nintendo system to have a memory card. However, it's actually called the Controller Pak by many games. It plugged into the controller's accessory slot. The Controller Pak, however, wasn't necessary for many games as the cartridges that the system used allowed for data to be directly saved onto it (much the same as the cartridges for the Game Boy). Many games just used the Controller Pak for storing non-main game data. An example of this was Mario Kart 64, where all race data was saved onto the game, but Ghosts from time trials were saved onto the Controller Pak. Most Controller Paks could only hold up to 123 pages of memory. However, there was an unofficial accessory released in 1996 to allow users to transfer saves onto their computer known as the DexDrive, bypassing the need to buy multiple Controller Paks.
Unlike the Nintendo 64, the Nintendo GameCube was very dependent on the memory card. As with all disc using games, data was either saved on internal memory or on a memory card. The GameCube has no internal memory and had two slots for memory cards. Games played without the memory card inserted act as if it were being played for the first time and data cannot be saved.
There have been three different kinds of GameCube memory cards that were released. The first could store up to fifty-nine blocks; the second could store up to 251; the third could store up to 1,019.
One block is roughly 8KB.
The Wii is less dependent on memory cards. It has files of internal memory for saving game data (unlike the GameCube). It does however, have a slot for an SD card (also used in cameras and other devices). SD cards can store the Wii's channel data and some game data. Pictures taken on a camera saved on an SD card can be displayed on the Wii's Picture Channel. Some games also use the SD card to store non-main game data (like Mario Kart 64 did on the N64). An example of this was the snapshots that can be taken in Super Smash Bros. Brawl. These snapshots can be saved onto either the Wii itself or the SD card.
Before Wii System Menu 4.0 was released, Wiis could only use standard SD cards, which could store up to 2 GB of memory. System Menus 4.0 and later gained the ability to use SDHC cards that could store a maximum of 32 GB of memory. However, most games do not support SDHC cards, such as Super Smash Bros. Brawl.
One block is roughly 128KB if the 512MB capacity of the internal memory were split into 256MB for user use and 256MB were for system use and the user gets 2048 blocks ideally.