Digital ballasts use a microcontroller (integrated circuit) to control the ballast's operation whereas the "non" digital ones rely on a feedback circuit comprised of capacitors, resistors, etc and no integrated circuits. This "non" digital type generally has no hot-restrike protection, meaning that if you turn off a "hot" HID bulb and then turn it back on you can cause damage to the bulb. Slim line ballasts come in digital and "non" digital as well.