What are LEDs?

LED stands for light-emitting diode. LEDs are light sources that become illuminated by the movement of electrons through a semiconductor material. The simplest packaged LED product is the lamp, or indicator. The basic structure of an LED indicator consists of the die, a lead frame where the die is actually placed, and the encapsulation epoxy, which surrounds and protects the die and disperses the light.  LEDs are used as indicator lamps in many devices, and are increasingly used for lighting. Introduced as a practical electronic component in 1962, early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet and infrared wavelengths, with very high brightness......... more from Wikipedia

There are two main types of LED - 
Low-Powered LEDs used for localised lighting, signs, power buttons, small warning lights.

High-Powered LEDs used to illuminate a larger area. LED lighting normaly uses multiple LEDs inside a lamp fixture to produce white light. High power LEDs are of a more robust construction and will usually have a heat sink or pad.

Comparisons

	LED lighting products use light emitting diodes to produce light very efficiently. The movement of electrons through a semiconductor material illuminates the tiny light sources we call LEDs. A small amount of heat is released backwards, into a heat sink, in a well-designed product; LEDs are basically cool to the touch
	In a CFL, an electric current is driven through a tube containing gases. This reaction produces ultraviolet light that gets transformed into visible light by the fluorescent coating (called phosphor) on the inside of the tube. A CFL releases about 80% of its energy as heat
	Incandescent bulbs create light by passing electricity through a metal filament until it becomes so hot that it glows. Incandescent bulbs release 90% of their energy as heat

The measurement of light is a little complicated because various devices for producing it have differing efficiencies

Light is measured in lumens.
LUX is an abbreviation for Lumens per square meter.
Foot-candles equal the amount of Lumens per square feet of area
One candlepower equivalent equals 12.57 lumens

Conversions are done on the basis of the amount of light LEDs emit (sometime in lumens or lux units). i.e 
a 60W bulb gives out 630 Lumens 
a 9W cfl gives out 565 lumens 
a 10W Led gives out more than 700 lumens

Miscellaneous Information

Visible light falls in the region of 400–700 nm
('nm' a unit of length in the metric system, equal to one billionth of a metre)