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. 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.
The primary feature in choosing a regulator is usually the Output voltage as the main purpose of one is to regulate the voltage to a specific value such as 3.3V. Its accuracy to this voltage across temperature and load current is another consideration as is its maximum load. The dropout voltage value will be particularly important with battery applications.