The Secure Digital, often shortened to SD Card, is an item used in the Wii, Nintendo DSi, Nintendo 3DS, Wii U and SD Card Memory Card Adapter for Nintendo Gamecube.
For Dōbutsu no Mori e+ users can use a SD Memory Card Adapter to take screenshots of the game.
For the Wii, it is used to save pictures taken on in-game cameras and Wii Shop Channel games. City Folk and any version of the Wii Menu before 4.0 don't support SD Cards larger than 2 GB.
For the Nintendo DSi, it is used to save either the DSiWare Applications, or for photos taken on the DSi's internal and external cameras.
For the Nintendo 3DS, it is used to store the downloadable games from the Nintendo eShop such as Animal Crossing: New Leaf and also save pictures taken in-game.
For the Wii U
it is used for Wii Mode and also to transport photos from 'Animal Crossing: New Leaf to Animal Crossing Plaza to be posted on Miiverse.
Prototype of the SD Memory Card Adapter
Player being asked it they want to save the screenshot on to a SD Card in Dōbutsu no Mori e+
SD Cards mostly used for Wii