CFL, or compact fluorescent light bulbs, produce light when electricity is routed through a small tube that contains argon and mercury. When the electricity mixes with these gases, it produces light. CLF light bulbs use less electricity than incandescent bulbs.
Despite using less electricity in the long run, CLF bulbs use more when they're first turned on. This is because it takes from 30 seconds to 3 minutes for the gases in the bulb to completely heat up and emit their full light.
When purchasing a CFL bulb, be sure to look for the ENERGY STAR label to save money and buy the right size bulb for the fixture.