A diode can be thought of as the electronic version of a one-way valve. By restricting the direction of movement of charge carriers, it allows an electric current to flow in one direction, but essentially blocks it in the opposite direction.
A semiconductor diode has a single p-n junction. If it is connected to a current source, with the p region connected to a negative pole, and the n-region to a positive pole, the holes will be attracted towards the negative pole, and the electrons to the positive pole. This enlarges the depletion layer, which makes the insulated space larger, stopping current flow across the junction.
If the current source is reversed, lots of holes flow across the junction towards the negative pole, and electrons travel in the opposite direction towards the positive pole. The p-n junction floods with charge carriers, the depletion layer disappears, and with it the insulator effect. In this direction, the diode lets current flow.
So, using conventional current flow, a diode lets a low-voltage current flow through it if current flows from its p-side to its n-side, but stops current flowing through it, from its n-side to its p-side.
A Zener diode is designed to block current flow through it, but if the voltage of the current source is large enough, it can force current to flow through the diode. This is called breakdown. As breakdown voltage is reached, the Zener diode’s resistance suddenly collapses. It lets a large current flow through it, without damage.
Because Zener diodes respond to certain voltage changes like switches, they are used in voltage regulators.
Light-emitting diodes, or LED’s, emit light when they are connected in a forward direction.