HD15 is the name for the connector commonly used for VGA Monitors for computers. It is a D-shaped (or "D Shell") connector which differs from DB15 in that it is the size of a DB9 connector but with three rows of pins to accomodate 15 connections (plus a shield ground.) This connector was also used, among other purposes, for the controller connector for Atari's Jaguar console video game system.
HD15 pins are arrayed in three rows of five connections each. On the female side, the top row (on the "big" side of the D-shaped connector) is pins one through five, counting from the right to the left. The row beneath it is 6 through 10, and the third row (at the narrow side) is 11 through 15, all right to left.
The primary advantage of HD15 over DB15 for video (as used by earlier Macintosh computers) is that DB15 is also used for PC-type Joysticks and for AUI ethernet. It makes little sense to reuse a connector type on the back of a computer for a purpose which is completely different and which will likely destroy something if plugged in incorrectly. Otherwise, it seems to stay plugged in via friction about equally well as a DB15 (though generally one is meant to screw the cables down) and it is much harder to straighten bent pins in an HD15 due to the density.