Ballasts are one of the most common things you never see. The average consumer just doesn't have to deal with them, as they are either used for large-scale installations of light bulbs, or they're mini-size and built in to compact fluorescent light bulbs. If you do happen to see a regular ballast, it would look like a metal box with wires of various lengths coming out of it. This odd-looking contraption is filled with electronics that control the starting and operation of fluorescent and HID light bulbs.
As they are bulky and unattractive, they are kept in out of sight positions, usually direct behind or below the lighting installations. Even though they are kept in the dark, they're still extremely important. If it were not for ballasts controlling our fluorescent and HID lights, we'd be out of street lights, grocery store lights, compact fluorescent home lights; the list goes on. Countless installations and light bulbs would be rendered useless without ballasts to control them.
Ballasts contain firmware which controls algorithms to supply varying lamp currents during start-up and operation. A digital HID lamp ballast uses microprocessors to control and regulate a certain number of HID lamps connected to it. There is a lot of technical jargon related to the use and upkeep of ballasts that I won't go into. I'm just glad there are people who will!
Electronic ballasts are what make compact fluorescent light bulbs work. Without them, CFLs would flicker and consume far more energy due to unregulated current. It's great that we have technicians and engineers able to design and create such electronics; without them I would not have access to my daylight compact fluorescent light bulbs, our streets wouldn't be lit, nor any of the big stores (grocery or otherwise) you're used to going to!