Secure Digital (SD) cards are flash memory cards developed for use in portable devices such as digital cameras, Mobile phones , PDAs and GPS systems etc.
It is the Flash memory card introduced in 1997 by Siemens AG and SanDisk based on the NAND based flash memory of Toshiba. It is small in size than the older Compact flash and has a size of 24mmx32mmx1.4mm just like a post stamp. Newer versions of MMC allow transfer of 4 or 8 bits at a time. MMC can be used in most of the systems that support SD cards.