Secure Digital (SD) is a flash (non-volatile) memory card format developed by Matsushita, SanDisk and Toshiba for use in portable devices, including digital cameras, handheld computers, PDAs and GPS units. As of 2007, SD card capacities range from 8 MB to 16 GB. Several companies have announced SD cards with 32 GB. Cards with 4-32 GB are considered high-capacity. The format has proven to be very popular. However, compatibility issues between older devices and the newer 4 GB and larger cards and the SDHC format[1][2] have caused considerable confusion for consumers.

      SD cards are based on the older MultiMediaCard (MMC) format, but have a number of differences: ?The SD card is asymmetrically shaped in order not to be inserted upside down, while an MMC would go in most of the way but not make contact if inverted. ?Most SD cards are physically thicker than MMCs. SD cards generally measure 32 mm ?24 mm ?2.1 mm, but can be as thin as 1.4 mm, just like MMCs (see below). ?The contacts are recessed beneath the surface of the card (like Memory Stick cards), protecting the contacts from contact with the fringes. ?SD cards typically have higher data transfer rates, but this is always changing, particularly in light of recent improvements to the MMC standard. Devices with SD slots can use the thinner MMCs, but the standard SD cards will not fit into the thinner MMC slots. SD cards can be used in CompactFlash or PC card slots with an adapter. miniSD and microSD cards can be used directly in SD slots with a physical interface adapter. There are some SD cards with a USB connector for dual-purpose use, and there are card readers which allow SD cards to be accessed via many connectivity ports such as USB, FireWire, and the parallel printer port. SD cards can also be accessed via a floppy disk drive with a FlashPath adapter.