An LED is a light-emitting diode that has a semiconductor light source. They emit light whenever a voltage is applied to it. LED’s were introduced in 1962, but have come a long way since then.
They have been used in many devices, as indicator lights, as well as in other lighting. They now are available across visible, ultraviolet and infrared wavelengths, and have very high brightness.
LED technology has progressed to the point where it is viable for general lighting applications.