A diode is a component that allows current to easily flow through it in one direction (from the anode to the cathode). Diodes are commonly found in power supplies and converters.

A light emitting diode (LED) is a popular sub-category of diodes that light up as current flows through it. LEDs work only when they're connected in the correct orientation.

A diode's cathode (or "output" end) is marked with a white band. An LED's cathode is normally the short lead, and the plastic bulb has a flat edge.

In order for the diode to turn on and let current through, the voltage at the anode (input) has to surpass a threshold. This is known as the forward bias or forward threshold voltage. The threshold voltage can be 0.6V-5V.

If voltage is applied to the cathode (output) end, the diode will resist current flow up to a certain voltage limit. This is known as the breakdown voltage. If the voltage surpasses the breakdown voltage, the diode will break down and act like a short circuit.

When this happens, the diode is usually toast. Zener diodes, however, are specifically designed to breakdown repeatedly without being damaged.

Diodes (including LEDs) can break either as an open-circuit or a short-circuit. If too much current flows through the "forward" direction, the diode will burn out like a fuse, creating an open-circuit. If the diode is accidentally installed in reverse, it may short-circuit.