The gas in the tube glows with light in the ultraviolet spectrum when electricity from the ballast energizes it. The ultraviolet light excites the phosphor coating on the inside of the lamp, emitting visible light. What we are most familiar with in our homes are incandescent bulbs. They work by using electricity to heat a tungsten filament until it glows white-hot.
Fluorescent lamps project a more diffuse light, but do it without wasting energy on heat.
I almost completely outfitted my own home with compact fluorescents eight years ago during a local utility's rebate program. The initial results were disappointing. The bulbs did not last even half as long as the incandescent lamps mounted next to them in identical fixtures on the same circuit. In the first six months, all but one of the original compact fluorescents failed, and some had to be replaced more than twice. It was hard just to read a magazine while sitting at the kitchen table, despite six ostensibly 75-watt equivalent compact fluorescents in the ceiling.