The GameCube features two ports that accommodate memory cards for saving game data. The three official memory card sizes are: 59 blocks (4 Mbit/512 KB, gray card), 251 blocks (16 Mbit/2 MB, black), and 1019 blocks (64 Mbit/8 MB, white). Third-party memory cards are also available.
Source: Wikipedia, "Gamecube", available under the CC-BY-SA License.
The memory cards are compatible with the GameCube and some models on the Wii. Wii models that are compatible have a lift flap which reveals controller ports for the GameCube, as well as slots for the memory cards themselves.