L.E.D. Iight Emitting Diode.
A diode glows when electricity is added to it. Think of a light bulb with the filament inside. When you add electricity, the filament heats up and gives light.
LEDs are almost the same thing except they don't have filaments. The light is caused by the lightning bolt between the two contacts inside the diode.
It takes a very small amount of juice to activate a diode and they give off much more light than the electricity they use.
If I put 10 volts into a diode, it will give off 100 bits of light. If I add another diode and put the same 10 volts, they will both put out 99 bits of light. They are ganged components. The more of them you have together, the more intense the light for almost the same amount of electricity. This is why they are so popular. They run on a small amount of juice and still give nearly as much light.
Diode quality has a lot to do with clean standards. The more pure the doping is (doping=coating a diode), the better quality of light and the more of it.
So basically, LED's are very efficient makers of light and the main thing to look for is the build quality. Does it use quality parts, or is it just slapped together?