Using LEDs

LEDs, which is stand for Light Emitting Diodes, are now very widely used in almost every area of electronics, mainly as indicator and display devices - in effect, 'solid state lamps'. They’re very well suited for this kind of use, because they are physically quite rugged and hence much more reliable than filament-type incandescent lamps.
They also run much cooler and are much more efficient, requiring far less electrical power input for the same amount of light output. Other common uses for LEDs are as a source of either visible or infra-red light, transmitted as a carrier for data and other information over short ‘line of sight’ distances. 

A LED is basically just a specialised type of P-N junction diode, made from a thin chip of fairly heavily doped semiconductor material. When it is forward biased to reduce the potential barrier provided by the junction’s narrow depletion layer, electrons from the semiconductor’s conduction band can combine with holes from the valence band, releasing sufficient energy to produce photons of light. Because the chip is thin a reasonable number of these photons can leave it and radiate away as its light output.