LED: Definition and Much More from Answers.com

LED
(Light Emitting Diode)A display and lighting technology used in almost every electrical andelectronic product on the market, from a tiny on/off light to digitalreadouts, flashlights, traffic lights and perimeter lighting. LEDs arealso used as the light source in multimode fibers, optical mice andlaser-class printers.
LEDs Vs. LCDs
In the early 1970s, red LEDs were used in the first digital watches,but were superseded by lower-power LCDs within a few years. LEDs stilluse more power than LCDs, but less power than incandescent bulbs. Theyalso last for decades and are virtually indestructible.
LEDsand LCDs coexist on countless devices where the LEDs provide the statuslights, and the LCDs display data. In addition, white LEDs can providethe backlight for LCD screens. SeeLCD.
Several Colors
LEDs are semiconductor diodes that typically emit a single wavelengthof light when charged with electricity. Originally red, today, severalcolors can be generated based on the material used for the tips of theprobes. Aluminum indium gallium phosphide (AlInGaP) is used for red andyellow. Indium gallium nitride (InGaN) is used for green and blue, andwith the addition of phosphor, for white light as well. SeeOLED,IRED,LED printer,fiber optics glossary andNixie tube.
 

An LED Unit
The LED is the semiconductor die itself, which sits in a reflective cup that acts as a heat sink and reflector. When voltage is applied to the LED, electrons and holes in the two semiconductor layers are attracted to each other at the junction. When they combine, they create photons.
 

LED Vs. LCD
LEDs and liquid crystal displays (LCDs) are widely used in combination as in this printer control panel. Alphanumeric readouts can be LED or LCD, but the billions of on/off indicators found on countless products such as this green Ready light are LEDs.
 
 

The First LED Watch
This Synchronar 2100 was the first solar powered watch. It also has the distinction of being the first LED watch. Models were already working prior to the announcement of Hamilton‘s famous Pulsar LED watch in 1970. (Image courtesy of the private collection of Peter Wenzig.)
 

Change the Mood
LEDs provide much more flexibility than ordinary lights; witness this Los Angeles bedroom with illumination by Color Kinetics. With a flick of a switch, you can change the ambience. (Image courtesy of Color Kinetics, Inc.; Design and photo by Steven Cordrey)
 

Rockin‘ Stairs
This staircase in the Westin Hotel in Shanghai is illuminated by LEDs from Color Kinetics. It can cycle through a rainbow of colors or become a light show synchronized with music. (Image courtesy of Color Kinetics, Inc.; Design by Light Directions Hong Kong; Photo by Friendly Light)