The xD-Picture Card is a flash memory memory card format developed and introduced to the market in July 2002 by Olympus and Fujifilm. xD cards are used exclusively in Olympus and Fujifilm digital cameras, and are available in a range of sizes, from 16 MB to 1 GB. They primarily compete with formats such as Secure Digital Card (SD), CompactFlash (CF), and Sony Memory Sticks. xD stands for "extreme digital."
Typically, an xD card is used as storage media for a digital camera, in a form that can easily be removed for access by a PC. A digital camera would use an xD card for storing image files. With an xD reader a user could copy the pictures taken with the digital camera off to his or her computer. Modern computers, both laptops and desktops, rarely have built-in xD slots even when other formats are supported, due to the general lack of popularity xD suffers from. However, PCMCIA and CF adapters are available for xD-Picture cards, enabling them to be used in readers and cameras which do not have native support for the xD format.
The xD card offers a single advantage over its direct competitors: size. An xD card is 20 by 25 by 1.78 mm and weighs about 2.8 g, making it significantly smaller than the matchbook-sized CF card and about half the size of the more-common SD card. As far as size goes, it is much closer to miniSD or RS-MMC, both which are generally used in cell phones rather than digital cameras.
xD cards offer quite a few disadvantages, however, in comparison to their more-popular competitors.
The most significant disadvantage for a consumer is their limited availability and support. xD cards are only used in Olympus and Fuji cameras. xD card readers are slightly more difficult to find, as well. This means that, unlike SD or CF, someone with multiple devices that use memory cards will be unable to reuse xD cards in any of their other devices.
Another significant disadvantage is their greater cost, due to the restrictive (and expensive) licensing required to manufacture them. In general, an xD card costs approximately twice as much per-megabyte as an SD card, and about two-and-a-half to three times as much per-megabyte as a CF card.