Compact Fluorescent Lamp (CFL)

CFL is a type of energy-saving lamp, which produce light differently than incandescent bulbs. In an incandescent, electric current runs through a wire filament and heats the filament until it starts to glow. In a CFL, an electric current is driven through a tube containing argon and a small amount of mercury vapor. This generates invisible ultraviolet light that excites a fluorescent coating (called phosphor) on the inside of the tube, which then emits visible light.

Source: http://www.energystar.gov/index.cfm?c=lighting.pr_what_are#what_are