Zener diode

Zener Diodes are used to supply a reference voltage or to prevent voltages from rising above a set threshold. They act like regular Diodes when forward biased (anode more positive than the cathode), but will exhibit a mostly constant voltage across them when reverse biased.

You generally use them when you need to generate a stable voltage at some point in the circuit from a source voltage that you cannot guarantee to be precise enough for your purposes, or to limit voltage to a part of the circuit. They are commonly available in values from 1V to 200V+. The voltage they are rated for comes with a Temperature coefficient that tells you how much variance in reverse breakdown voltage occurs due to Temperature change. If you need your circuit to be highly Temperature stable and don't want to compensate with additional components, the 5.6 V variety has the lowest Temperature coefficient.