A fluorescent light uses a tube of excited mercury gas to illuminate a phosphorescent coating, producing visible light. Fluorescent lights use less energy, produce less heat and tend to last longer than incandescent light bulbs.
Inside a fluorescent bulb is a gas containing mercury. When electricity flows into the tube, it ionizes this gas causing it to emit photons of ultraviolet light. These photons are invisible to the naked eye, but when they strike the phosphor coating on the inside of the tube, they transfer energy to it and cause it to illuminate.
The small amount of mercury present in fluorescent and CFL lamps means that one must proceed with care when cleaning up after a broken bulb to avoid potential mercury exposure.