Secure Digital (SD) is a non-volatile memory card format developed by theSD Card Association (SDA) for use in portable devices. The SD technology is used by more than 400 brands across dozens of product categories and more than 8,000 models, and is considered the de-facto industry standard.

Secure Digital devices comprise cards of the same physical dimensions but different communication protocols. The Standard-Capacity (SDSC) card family (the original SD card), stores up to 2 GB (but see below). The High-Capacity (SDHC) card family has a capacity of 4 GB to 32 GB.[3] eXtended-Capacity (SDXC) card family has a capacity starting above 32 GB with a maximum of 2 TB.[4][5] The SDIO family implements input/output functions rather than just data storage.

The current Secure Digital specifications now also comprise three different form factors: the original size, the "mini" size, and the "micro" size . Electrically passive adaptors allow the use of a smaller card in a host device built to hold a larger card. There are many combinations of form factors and device families. SD, miniSD, and microSD are members of the SD family. SDHC, miniSDHC, and microSDHC are members of the SDHC family. SDXC and microSDXC are members of the SDXC family. SDIO and miniSDIO are members of the SDIO family.

The newer (higher-capacity) communication protocols provide backward compatibility, but older host devices do not recognize the newer card families. The existence of cards of different families but with the same physical size has confused customers.http://en.wikipedia.org/wiki/Secure_Digital - cite_note-5 (Most incompatibility issues could be resolved by updating the firmware of the host device, but vendors of host devices are more likely to support newer card families by releasing an entirely new product.) In addition, each family specification guarantees a maximum device capacity that the host device is required to support. For example, the SDSC specification set an upper limit on number of blocks that suggested a 2 GB maximum. Some SD cards provided double this amount (though 4 GB would now be achieved using SDHC) by cleverly doubling the block size. Though these 4 GB devices were technically SD-compliant, some SD host devices did not support them.