See CompactFlash.

A flash memory format introduced in 1994 by SanDisk Corporation, Milpitas, CA (www.sandisk.com). At 36.4 x 42.8 x 3.3 mm, CompactFlash (CF) cards are larger than other flash cards, but are popular storage in digital SLRs, which more easily accommodate their size than small point-and-shoot cameras.

Appearing to the camera as a hard disk, CompactFlash (CF) uses a 50-pin PC Card interface. CF cards plug into 50-pin sockets in digital cameras or a 68-pin Type II PC Card slot in a laptop with a 50-to-68-pin adapter. The technology supports both 3.3v and 5v circuits.

Type II Cards Are Thicker
Type II CompactFlash (CF2) cards are the second-generation format, which increases the thickness from 3.3 to 5mm. The extra thickness allows for more electronics; for example, IBM's Microdrive houses a hard disk in the CF2 format. In 2004, Pretec Electronics Corporation (www.pretec.com) introduced a 12GB CF2 memory card. See Microdrive.

What Does the X Mean?
The write speed of CompactFlash cards is designated as a multiple of 150 kilobytes per second. For example, 40x = 6MB/sec; 60x = 9MB/sec; 66x = 10MB/sec and 80x = 12MB/sec.