Secure Digital (SD) is a flash (non-volatile) memory card format developed by Matsushita, San Disk, and Toshiba for use in portable devices. Today it is widely used in digital cameras, handheld computers, PDA's mobile phones, GPS receivers, and video game consoles. SD card capacities range from 8 MB to 2 GB and from 4 GB to 32 GB for SDHC cards as of 2008.

Design and Implementation

An SD card, mini SD card, and micro SD card from top to bottom cards are based on the older MultiMedia Card (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 MMC's. SD cards generally measure 32 mm × 24 mm × 2.1 mm, but can be as thin as 1.4 mm, just like MMC's (see below). 
The contacts are recessed beneath the surface of the card (like Memory Stick cards), protecting the contacts from contact with the fingers. 

Memory Cards

A memory card or flash memory card is a solid-state electronic flash memory data storage device used with digital cameras, handheld and Mobile computers, telephones, music players, video game consoles, and other electronics. They offer high re-record-ability, power-free storage, small form factor, and rugged environmental specifications. There are also non-solid-state memory cards that do not use flash memory.

There are many different types of memory cards and jobs they are used for. Some common places include in digital cameras, in game consoles, in cell phones, and in industrial applications. PC card (PCMCIA) were among first commercial memory card formats (type I cards) to come out in the 1990s, but are now only mainly used in industrial applications and for I/O jobs (using types I/II/III), as a connection standard for devices (such as a modem). Also in 1990s, a number of memory card formats smaller than PC Card came out, including CompactFlash, SmartMedia, and Miniature Card. In other areas, tiny embedded memory cards (SID) were used in cell phones, game consoles started using proprietary memory card formats, and devices like PDAs and digital music players started using removable memory cards.