Diodes are electronic components that only let electricity flow in one direction in ideal conditions. An exception is a Zener diode, which does only allow electricity to flow in one direction until a voltage threshold is reached, and then it can pass through the diode.
How a Diode Works by Hana Shoib
Light Emitting Diodes
Commonly referred to as their acronym "LED", light emitting diodes are diode components in which light emission is a byproduct of the electricity passing through the diode element. They come in a variety of colors, and are much more energy efficient than incandescent light bulbs.