If you build an astronomy device and want to control it with some awesome PC software, then you need to write an ASCOM (AStronomy Common Object Model) driver. There are great resources available for writing a single ASCOM driver for your DIY astronomy equipment. Check out the video on the ASCOM website for a guide on writing a single-device driver! But say, for instance, you have two devices (a Telescope and a Focuser) and would like to control them with one microcontroller. Or say you want to control your telescope with Astrophotography Toolkit AND PHD2 at the same time. Then you need to write a Driver Server!