A Linear Voltage Regulator ensures the output voltage from the circuit is maintained at a specific voltage. The device acts as a variable resistor, continuously adjusting to maintain a constant output voltage. The regulated voltage must be lower than the input voltage. Linear voltage regulators are step-down converters which means that the output voltage is lower that the input. Simple voltage regulator contains a Zener Diode and a resistor but more complicated versions include elements like a voltage reference and error amplifiers.
A more specific type of regulator are Low Dropout Voltage Regulators (LDO) which can regulate voltage even when the power supply voltage is very close to the supply voltage. They have low noise and good ripple rejection making them crucial to maintaining a stable power supply in any power-management system.