Incandescent Light Bulb: Your typical household light bulb, a glass ball with a tiny metal filament inside that conducts electricity and produces light. These bulbs are highly inefficient, producing only around 17 lumens/Watt. Most of this energy loss is due to the heat given off by these bulbs. Typical life expectancy? About 1,000 hours.
Compact Fluorescent Light (CFL) Bulb: Instead of a metal filament, electricity is passed through mercury vapor within a glass tube, emitting ultraviolet light. This ultraviolet light then reacts with a phosphor coating in the bulb, causing it to glow. Though CFL's don't produce as much heat as incandescents, there is still energy loss through the process of creating ultraviolet light and converting it to visible light. Most CFL's produce about 60 lumens/Watt, and last nearly 8,000 hours, so their cost (between 3 and 10 times that of a typical incandescent) is offset by energy & replacement savings.
Light Emitting Diode (LED) Bulb: With an efficiency similar to that of the CFL and a life-expectancy of about 30,000 hours, LED's sound like the way to go, but don't be so sure. LED's tend to be "directional light." This means that, where incandescents and CFL's produce light omni-directionally, LED's focus light in one spot. This makes them great for spot-lighting driveways and small porches, but not necessarily for lighting rooms. Greater cost is another downfall, running at about $30-$50 each.
Now you are prepared to make an educated decision on the replacement of your old, inefficient incandescent light bulbs, whether it be LED spotlights for your driveway or CFL's for your bedroom.