How do diodes work?

A diode has two terminals: an anode (positive side) and a cathode (negative side).
The diode conducts current when the voltage at the anode is higher than the voltage at the cathode by more than a value specified in a datasheet (roughly 0.7 V in the case of silicon pn junction diodes). It does not conduct current when the difference between the anode and cathode potentials is less than this value. Such action is called rectification. This diode characteristic can be used for rectifier circuits that convert direct current to alternating current (AC-DC conversion), reverse battery protection (reverse current protection), and radio wave detection.
The bias state in which the anode terminal is higher than the cathode terminal is called forward bias, and the state in which it is lower is called reverse bias.
When a reverse bias is applied to the diode and this voltage is increased, current suddenly flows at a voltage called breakdown voltage. Breakdown voltage is almost constant irrespective of current. Utilizing this characteristic, diodes are also used in constant-voltage circuits and nowadays for ESD and surge protection. However, typical diodes are subject to degradation and permanent damage when they are operated in the breakdown region. It is therefore necessary to use dedicated diodes such as Zener diodes or ESD protection diodes for these applications.