Often abbreviated as simply "CF," Compact Flash is a type of flash memory. Compact flash cards are most commonly used for storing pictures in digital cameras, but are also used in devices such as PDAs and portable music players.
There are two types of Compact Flash cards, creatively named "Type I" and "Type II." Type I cards are 3.3 mm thick, while Type II are 5 mm thick. IBM makes a "MicroDrive" card that has the same dimensions as a Type II CF card, but uses an actual hard drive construction rather than flash memory. Compact Flash cards originally could only store a few megabyes of data, but now can store several gigabytes. The new CF+ standard can store a possible 137 GB of data. I don't know about you, but I'd think about backing up that information before throwing the card in my pocket.