A voltage regulator is a device which takes a high or variable input voltage and puts out a lower and stable output voltage. In its most common modern form, a voltage regulator works with a feedback loop that controls the output of a transistor. The feedback circuit compares the output voltage with that produced by a zener voltage reference to determine the correct gate current to the output transistor.
In tube equipment, the voltage reference will usually be a special mercury vapor tube, and the circuit itself may be a simple clamp or driven by a larger thermionic valve.
On an automobile, the voltage regulator is either an electromechanical or solid-state device which senses the battery voltage and switches the alternator output on or off to maintain the proper voltage on the electrical system. This is usually 13.8 volts. If your battery is not charging, check the alternator and voltage regulator. If the voltage is going too high, replace the voltage regulator immediately, or it will zorch your battery by overcharging it! Usually, if the battery is failing to charge and the alternator light is not on, the connections or the voltage regulator are at fault, as it also supplies the +12 volts to the dashboard idiot light if it's not getting power off the alternator. Electromechanical regulators are adjustable; solid state ones are a non-user-serviceable, potted, black box.
In a power supply circuit, the voltage regulator may be a single integrated device, such as the LM7805, with or without external drive transistors, or it may be composed of discrete components. The regulator may or may not be adjustable.