It used to be so easy; you only had to know not to put a 100 watt bulb in a 40 watt lamp. Now, shopping for your next light bulb may introduce you to some new concepts and terminology. But have no fear, we are here to sort it all out.

From Energy.gov
The incandescent light bulb (the old kind) has been found to be very inefficient: it releases 90% of it’s energy production as heat, not light. Many wattages of these bulbs will no longer be manufactured. However, more energy efficient and greener choices are readily available. This guide (as well as the links to other guides) will help you understand the varieties of new bulbs.
Energy efficient bulbs come in a variety of sizes, strengths, and colors to suit a variety of needs. Most of these bulbs have a longer lifespan than the average incandescent bulb. They are produced in a spectrum of hues and colors: the light warm and incandescent-like, bright and daylight-like, or cool and sky-like. Even tinted bulbs and black lights come in energy efficient options! They are also much more expensive to buy (though cheaper to use) than the old incandescent bulbs, so you want make sure you buy the right ones.
Energy efficient bulbs:
Halogen incandescents emit a light very similar to incandescents, but increase the energy efficiency of the bulb by 28%.
Compact fluorescents (CFLs) now have a softer, more natural color, use 75% less energy than incandescents, and last about 9 years.
LEDs use 80% less energy than incandescents, have a life-span of about 20 years, and are available in a variety of colors.
In the past, we have commonly shopped for light bulbs by their wattage only. The watt measures how much energy is used. Now, in addition to watts, you will also want to consider Lumens and Kelvins.
Lumens represent how much light is produced. The more Lumens, the brighter the bulb.
Kelvins measure the light’s color temperature. The lower the number, the warmer color of light; the higher the number, the cooler color of light.
For more information, visit NPR’s “Guide to Changing Lightbulbs”, Energy.gov, and The Department of Energy’s Lightbulb Tips and CFL Information.