An SD Card is a "Secure Digital Memory Card", a non-volatile storage device for portable electronics. They were initially available in 16MB, 32MB and 64MB sizes, but 512MB and 1GB models were announced at CeBIT 2003. Its transfer rate is either 10MB/s or 2MB/s.
The coolest thing about these devices is that they are incredibly small- the size of a minor postage stamp at about 32 millimetres long by 24 millimetres (~= 1 inch) wide and is about 2 millimetres thick.
The technology was developed by a consortium including Panasonic, Toshiba and SanDisk, and is used in Palm and Symbian devices. It was first used by the Panasonic ipalm PV-DC3000 digital camera.
A new range of dual-interface SD Cards from SanDisk have become available, called the "Ultra II". They work exactly like normal ones, except that they snap open, revealing a hinged section with a USB plug. You can slot it into a USB socket on a PC and read the card's contents like any other USB storage device.
Rival technologies include CompactFlash (CF), SmartMedia™ memory cards, the Sony Memory Stick and MultiMediaCards MMC, however none of these offer the security features of the SD Card. The card supports all current SDMI (Secure Digital Music Initiative) requirements and should support all future ones.