A D-Sub is the same a a VGA connector, which is the typical connector you use when connecting a monitor to a graphics card. If you have a CRT monitor, look at the end of the cable and you will see a 15 pin male D-sub/VGA connector.
A DVI connector is one found on most graphics cards made in the last 3 years, in addition to the traditional VGA connector. It provides what many people think is a better overall image quality to LCDs which support it. For mor information on DVI, go to the following website: