A voltage regulator is a circuit that produces a fixed output voltage and maintains the same voltage in all instances, regardless of changes in input voltage or load conditions.  Voltage regulators provide accurate output voltage.

 The voltage produced by alternators in vehicles may vary depending on their speed.  Voltage regulators are used in vehicles to prevent such varying voltages from adversely affecting the vehicle's power-using parts.
Voltage regulator circuits are usually constructed using transistors with zener diodes or ICs.