The LED ( Light Emitting Diode ) is found in just about every corner of our world that you can imagine.
The first consideration is that the LED is a Diode. Just like every other Diode current only flows in a single direction. When current flows in the correct direction the Diode emits Light.
The LED is comprised of a small chip inside of a clear or colored epoxy. It has two leads that allow connection to the Anode and the Cathode of the LED.