USB monitors use a new technology called DisplayLink. This technology allows VGA-quality video to be sent through a single USB cable, which is perfect for when you want to add an additional display but have run out of VGA ports.
If you're using your USB monitor for your in-car computer, you'll find it much easier to get connected - you don't need to supply a +12V power source (all the power is supplied by USB), and additional features such as touch screen and webcam are also sent through the same cable. If you're using it at home or in an office, you'll appreciate the portability and convenience of connecting a USB monitor to your desktop or laptop computer.
This kind of technology requires software drivers to work so there are minimum system requirements, unlike VGA. At present, Windows 2000/XP/Vista/7 are well supported, with Mac OS and Linux drivers currently being developed.