A type of removable flash memory card.
Like any flash memory card, it can be used to store various types of files, including photos, videos, music, or software.
SD is smaller than Compact Flash, but larger than miniSD, TransFlash, or RS-MMC.
SD is the same size as MMC, but thicker. Most devices with SD slots can also accept MMC cards, but not vice-versa.
SD cards are generally faster than MMC, but use more power.
There is an extension of the SD protocol known as SDIO that allows devices other than memory cards to be used in SD slots, such as cameras or wi-fi adapters.