DAT stands for digital audio tape. It refers to a type of tape recording technology, originally developed for music, which represents music on tape with numbers (digitally) rather than as analog sound waves (see digitize and A-to-D conversion). High-fidelity digital music requires a system that can record a great deal of data at high speed, and DAT technology measures up. Since the same requirements pertain to backing up a hard disk, DAT technology has been adapted for use with computers.
A DAT drive is the equivalent of a tape recorder, except that it records computer data instead of music onto dat cartridges. The cartridges are only about as big around as a playing card, and not much thicker than a standard audio cassette tape. DAT drives are fairly expensive compared to more common kinds of backup tape drives, and they're mainly used by businesses which can justify their higher cost in return for faster recording speed and greater storage capacity per tape.