This is the standard parallel interface on most computers. It was originally designed by the centronics group as a printer interface system.
It has now gone on to be used for scanners, removable drives, and robotics interface. Although pretty much superceded by the USB port (in a round-about way), it is one of the simplest places to connect something to your computer.

I would advise optically isolating any connections - and never drawing power for more than a few LED's from the port.