Regular D-SUB pins are somewhat large in diameter (still pretty small in the overall scheme of things). They fit into a connector that may have 24 pins over to rows. The same shape connector may be used, but will have 1.5 times the number of pins. The pins are typically closer together and they pins themselves are smaller in diameter. There are a higher density of pins in the same basic area. For example: The Garmin 430 uses a regular D-SUB 25 pin connector for the COM powers, grounds, dimmer, and audios. However, it uses two high-density D-SUB's for the GPS and NAV connectors. They are bigger connectors but have a large number of pins (The GPS connector has 78 pins). So in short, high-density means smaller diameter pins to fit more pins in a given area. Hope that helps.