CFL stands for "compact fluorescent lamp." As the name implies, CFLs are compact versions of the long, tubular fluorescent lamps that are commonly seen in offices, schools and institutional buildings worldwide. CFLs are becoming more familiar as they replace the standard, screw-in incandescent bulb.
According to the U.S. Department of Energy, CFL bulbs can save up to 75 percent of the energy used by an incandescent bulb. Though they cost more up-front, they last much longer, and they'll quickly pay for themselves through energy savings. They’re also cooler, fit in most lighting fixtures, and are available in warmer color tones (earlier models cast a cold, bluish light that some users found ugly). Some models are dimmable, but others can’t be used with a dimmer switch.
There is one serious downside to CFLs: they contain mercury, a dangerous element that's been associated with a range of serious health issues, including birth defects, loss of hearing, kidney damage and severe nervous system dysfunction, including brain damage. Because of the mercury in CFLs, they can't be thrown in the garbage if they stop working -- they must be recycled at a facility that accepts hazardous materials -- and if they break, it's not easy to cleaned up broken CFLs to avoid mercury contamination. For that reason, they might not be a good choice for some homes, especially if there are children present.
For more information, use this link to energy-efficient lighting descriptions.