What exactly is an LED?
Light Emitting Diode.
Light illuminating. Gives off some color (Wavelength) of light. This has been, in the past, generally a specific color wavelength -Red- (620-635 nano-Meters). Other colors have been produced in recent years Green (520-530nM), Blue (465-475nM) , Amber or Yellow (589-595nm) and even White (combination of Red, Green, and Blue nM).
Why use an LED?
They are extremely low power...generally in the low voltage realm