A voltage regulator is used to derive a stable voltage from a non-stable (or slow varying) voltage source. They are present in lots of electronic equipment where a stable supply is not present. When the required current is low, a zener shunt regulator can be used (is also cheap). Frequently the zener is now replaced by an integrated circuit that gives better stability (versus time and input voltage).
Series regulators are used when the current demand is higher. A series regulator requires a stable reference voltage that is frequently derived from an zener or IC parallel regulator. With the advent of fast switching mosfets and integrated control circuits, many linear series voltage regulators are replaced by their switching equivalents (efficiency). The switching regulator uses a zener or IC shunt regulator for generating its internal reference voltage. The shunt regulator circuit may be part of the integrated control circuit.