Diodes are semiconductor components that behave like one-way valves. They basically allow current to flow in one direction. Regular diodes will become destroyed if forced to conduct current in the wrong direction, but zener diodes are optimized to operate when placed backwards in a circuit.
Diodes are constructed from semiconductors such as silicon and germanium. The semiconductors are mixed with other elements such as boron and phosphorus in a process called doping. Zeners are made with silicon that is more heavily doped than regular diodes.
Ordinary diodes and zeners have a mark on their bodies. A diode is called forward-biased when current flow is from the unmarked to the marked side. It is called reverse-biased when current flow is the other way.
Zener diodes are placed into circuits in a reverse-biased position parallel to the load. A current limiting resistor is included in order to ensure that the power and maximum current specifications are not exceeded.
Diodes are used as rectifiers by changing AC current to DC current, by removing part of the signal. Some of their other numerous functions are as electrical switches and voltage doublers.
Zeners can perform the functions of ordinary diodes, but are most often used as voltage regulators for low current circuits, because they can maintain stable voltages under varying loads. They can protect circuits from voltage fluctuations, and so are found in devices such as power supplies and surge protectors.