What exactly is a shunt voltage regulator?
Both types of regulators can be considered as "variable resistors". The series regulators increase their resistance to lower the voltage on the load. The shunt regulators decrease their resistance to divert away current from the load.
As a result, unlike the series regulators, there has to be some resistance between the load / shunt regulator and the voltage source. Because of that, most high current regulators are series regulators.
Vs. a zener, a regulator, series or shunt, has much sharper knees, so their output voltage is far less dependent on external factors.