Secure Digital cards, or more commonly SD cards, are memory cards, commonly used for digital cameras. They are the Wii's and Wii U's main mode of memory expansion. They can be used to save snapshots, created stages, and replays from Brawl only.
The SD card is also used to replace files in Brawl with the "Smash Stack" hack. This hack exploits a bug in the custom stage loading ability in the game. This is a method used to commonly softmod the wii. Only SD cards with a capacity of 2 gigabytes or less are compatible with Smash Stack; SD cards with more than 2 GB of capacity can be used with Riivolution, a game patcher that requires softmodding of the Wii console.
SD cards are also used as additional memory cards for the Nintendo 3DS and the Nintendo DSi.