SD card
Secure Digital cards, more commonly referred to as SD cards, are a digital storage medium. Initially primarily used for digital cameras, the media has since attained widespread use as a storage medium, and it has since been used on the Wii, DSi and 3DS as a method of storage expansion. The Wii U also supports SD cards, but only uses them when running Wii games.The SD card holds memory from game saves and extra data for games.Just like the GameCube memory card.
In Super Smash Bros. Brawl
Standard save data for Brawl is not saved to SD cards, and instead, is stored on the Wii's own flash-based memory. SD cards, however, can be used to save snapshots, custom stages and replays from the Vault in order to be used or otherwise transferred on other Wii consoles. Of note is that because of the timing of the release of Brawl, the game does not support SDHC cards, a special type of SD card that supports over 2 gigabytes of data; while the Wii was later updated to take advantage of these cards, Brawl itself cannot be natively updated to do so.
Use in hacking
Through the use of an exploit in Brawl's Stage Builder, it is possible to load customised or otherwise hacked data from an SD card. In addition to allowing for the download of The Homebrew Channel, this also allows for the loading of replacement files, as well as gameplay mods such as Project M and Balanced Brawl. Through the use of the hack of Revolution, it is also possible to force Brawl to read SDHC cards.