A Jetdirect is a device made by Hewlett-Packard that allows a printer that does not have network capabilities to be shared on a local area network.
They usually come configured with one 10-baseT jack and one parallel port, but can be made with 10/100 Ethernet connections, a Token Ring adapter, and up to 3 parallel ports for connecting printers.
HP also makes an internal version of the Jetdirect, but it is less used due to the fact that it works mainly in only HP Printers. The external Jetdirects can be used to hook up any type of printer to the network.