The voltage regulator controls or regulates the alternator's output. Think of it as the brains of the charging system. It senses how much voltage is needed by your vehicle, then modifies the field current within the alternator so it puts out just the right amount of current. Too little current can allow the battery to run down while too much can damage it and other electrical and electronic components. When the regulator fails, the charging system usually ceases to function -- except in cases where the nature of the failure causes the alternator to run wild and overcharge the battery. In any event, the only cure for a dead or defective regulator is replacement.
In older vehicles, the regulator was a separate component usually mounted somewhere in the engine compartment. If this type of regulator failed, it could be easily replaced in a matter of minutes with a new one. But for the last decade or more, most regulators have been mounted in or on the alternator itself. This was done by the vehicle manufacturers to simplify wiring and assembly. It was also made possible by advances in electronics that allowed the regulator to be reduced in size to a small chip.
Charging systems that have a separate regulator mounted away from the alternator are referred to as "externally regulated" charging systems while those that have the regulator in or on the alternator are called "internally regulated" charging systems. On some vehicles there is no regulator at all! Voltage regulation is controlled by the engine computer.
Unfortunately, internally regulated alternators are packaged as a unit -- which means that if either component fails (alternator or regulator) both must be replaced. This is because internal regulators are not available separately (at least not to the general public or the typical service facility). Electrical shops and remanufacturers who rebuild alternators can get them and can replace the regulator separately if that's all that's wrong with the unit -- but they'll usually charge you the same as if you bought a rebuilt alternator.
The truth is, the high cost of labor today has made it impractical for most service facilities to fool around trying to rebuild or repair components like alternators, starters, carburetors, front-wheel driveshafts, transmissions and even engines. It's faster, easier and usually cheaper to simply replace the old unit with a new or remanufactured one than to try to overhaul or fix it. Besides, most new and remanufactured parts come with a guarantee.