Diodes are electronic devices that regulate the direction of flowing electrical current. Some diodes -- such as Zener diodes -- perform special functions such as voltage control. Of the several different types of diodes, rectifier diodes are the most common therefore, when instructions call for a "diode" without specifying what type, electricians assume that they are describing a solid-state rectifier diode.
The most common diode materials in use are silicon and germanium. Silicon and germanium are poor conductors of electrical current by themselves -- hence the name "semiconductor". Small amounts of boron, arsenic, or phosphorus go with pure silicon or pure germanium to make the semiconductor material conduct electrical current more freely. Germanium and silicon are solid at room temperature so electronic components fabricated from these materials are called "solid-state" devices.
Vacuum-tube rectifier diodes also exist; however, since silicon- and germanium-based diodes are (as of 2011) less expensive to produce and are more durable than vacuum tube-based diodes, vacuum-tube diodes are rarely used.
Rectifier diodes are the most commonly used diode type but other diodes come into play as well. These include Zener diodes, Schottky diodes, tunnel diodes, photodiodes, varicap diodes and light-emitting diodes. Each diode type has a different function. For example, Zener diodes control voltage, Shottky diodes work in switch circuits, light-emitting diodes are in lighting and video display circuits. Photodiodes are in light-detection circuits. A "varicap" diode is constructed like a diode but behaves as a variable capacitor.
Rectifier diodes act as a one-way check valve for electrical current. To conduct electrical current, the voltage at the diode's anode lead must be higher than the voltage at the diode's cathode lead. When the diode conducts current, the diode is said to be "forward-biased." If the voltage at the cathode is greater than the voltage at the anode, the diode will fail to conduct current. A diode in this condition is said to be "reverse-biased."
Zener diodes regulate voltage. Zener diodes allow electrical current to flow forward (from the anode to the cathode) through the diode. However, if reverse voltage across the diode -- that is, applied from the cathode to the anode -- exceeds the rated breakdown voltage, a Zener diode will also allow electrical current to flow backward through the diode. A Zener diode will keep the voltage across the diode at approximately the breakdown voltage, provided that the electrical current through the diode does not exceed the manufacturer's maximum rated current allowance.