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When forward-biased, zener diodes behave much the same as standard rectifying diodes: they have a forward voltage drop which follows the "diode equation" and is about 0.7 volts. In reverse-bias mode, they do not conduct until the applied voltage reaches or exceeds the so-called zener voltage, at which point the diode is able to conduct substantial current, and in doing so will try to limit the voltage dropped across it to that zener voltage point. So long as the power dissipated by this reverse current does not exceed the diode's thermal limits, the diode will not be harmed.
Zener diodes are manufactured with zener voltages ranging anywhere from a few volts to hundreds of volts. This zener voltage changes slightly with temperature, and like common carbon-composition resistor values, may be anywhere from 5 percent to 10 percent in error from the manufacturer's specifications.
Simply put, a zener diode is one that is designed to maintain a consistent voltage drop across itself when reverse-biased, regardless of circuit current, within the capacity of the diode. The voltage dropped is a specific design voltage and is basically equivalent to the zener voltage of the diode. Zener diodes are commonly used as voltage requlating devices. The reason that so many different values are offered is for versatility in circuit design.
Basically, zeners are hooked up backwards to take advantage of reverse breakdown voltage. Zeners aren't damaged by reverse breakdown, but regular diodes can be. This makes them a good choice in using their reverse breakdown voltage to limit another voltage in a circuit to a very specific value.
Zeners come with many different breakdown voltages because designers don't always want to limit the same voltage.
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