A type of removable flash memory card.
Like any flash memory card, it can be used for long-term storage of various types of files, including photos, videos, music, or software.
Full-size SD cards are smaller than Compact Flash, but larger than microSD, miniSD, or RS-MMC. MicroSD is the most common type of removable memory card in phones.
SD is the same size as MMC, but thicker. Some 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.