Also called a "dongle," it is an extremely effective software copy protection device that plugs into the serial, parallel or USB port of the computer. Parallel port keys also act as a pass-through to the printer or other parallel device. The nickname "dongle" was derived from seeing multiple keys "dangle" off the parallel port. Since serial and parallel ports have become legacy interfaces, new hardware keys mostly use the USB port.

No Key - No Run
In order for an application to run, it sends a code to the key, which responds by reading out a serial number. Each program package is tied to the unique number in the accompanying key, which ensures that the software runs in only one machine at a time because the key must be plugged in to work. See DMCA and PC Card dongle.