Google is your friend, as always. Here's a nice picture on Wikipedia.
Almost any UNIX workstation made before 1998 or so (And quite a few after, although VGA ports started turning up on the lower-end graphics options) used these for their monitor connections. (Often these systems would only have the 13w3 port on the system unit, with a cable breaking out the signals into four BNC coaxial connectors on the monitor end.)