Unlike standard fluorescents, they can directly replace standard incandescent bulbs. Compact fluorescent lamps work much like standard fluorescent lamps. They consist of two parts, a gas-filled tube and a magnetic or electronic ballast. The gas in the tube glows with ultraviolet light when electricity from the ballast flows through it. This in turn excites a white phosphor coating on the inside of the tube, which emits visible light throughout the surface of the tube.
Incandescent lamps work differently.
Most of the electricity they use heats a tungsten filament until it glows white hot. As a result, incandescent lamps generate far more heat than a fluorescent light. Since the light source is a single point, incandescents also project light further than compact fluorescents that project a more diffuse light. You should consider this when replacing incandescents with CFL's in high ceiling applications. Some compact fluorescents have the tubes and ballast permanently connected.