CF or Compact Flash was originally a type of data storage device, used in portable electronic devices. As a storage device, it typically uses flash memory in a standardized enclosure, and was first specified and produced by SanDisk in 1994. The physical format is now used for a variety of devices. There are two main subdivisions of CF cards, Type I and the slightly thicker Type II cards. There are two, soon to be three, main speeds of cards including the original CF, CF High Speed (using CF+/CF2.0), and an even faster CF3.0 standard that is being adopted as of 2005. The CF Type II slot is used by Microdrives and some other devices.
CF was among the first flash memory standards to compete with the earlier and larger PC Card Type I memory cards, and was originally built around Intel's NOR-based flash memory, though it switched over to NAND. CF is among the oldest and most successful formats, and has held on to a niche in the professional camera market especially well. It has benefited from having both a good cost to memory size ratio relative to other formats for much of its life, and generally having larger capacities available than smaller formats.
CF cards can be used directly in PC Card slot with a plug adapter, and with a reader, to any number of common ports like USB or FireWire. More impressively, thanks to its bigger size relative to the smaller cards that came later, many other formats can be used directly in a CF card slot with an adapter (including SD/MMC, Memory Stick Duo, xD-Picture Card in a Type I slot, and SmartMedia in a Type II slot, as of 2005) (some multi-card readers use CF for I/O as well).