A 42-watt CFL is a compact fluorescent light bulb that uses 42 watts of power per hour of use. CFLs are more energy efficient than traditional light bulbs because they use less energy to produce the same amount of light. CFLs use the same light fittings as traditional light bulbs.
Traditional light bulbs generate light by heating a tungsten filament, encased in glass. CFLs work differently to this, they have argon and mercury vapor, within a spiral shaped tube. When the vapor is heated, it emits ultra-violet light. A fluorescent coating on the inside of the glass, absorbs this energy, and emits visible light.