Background
SD cards can store data for a variety of devices such as cellular phones, computers and digital cameras. They come in several sizes, including regular SD, miniSD and microSD.
Capacity
Regular SD, microSD and miniSD cards can hold up to 2 gigabytes (GB), while their high-capacity (SDHC) counterparts can hold up to 32 GB.
Compatibility
MicroSD and miniSD cards can be used on a Wii if they're used with adapters to make them regular SD card sizes. SD cards up to 2 GB are compatible, but an upgrade to Wii Menu 4.0 can allow for the SDHC cards.
Game Uses
You can use SD cards to store game save data, Wii Channels and even virtual console games, which can be accessed directly from the card.
Non-Game Uses
On the Wii, SD cards can also transfer other compatible data such as sound files (MP3s and AACs) and digital photos to store on the console.