Why a zener diode is better than a simple diode?

A typical diode functions only in one direction (forward-bias). When subjected to reverse-bias, the junction will eventually break down under high enough voltage. A zener diode works the same way, but breaks down at a very specific reverse-bias voltage. This is called its zener voltage. Because of this property, a zener diode can be used to regulate the voltage in a circuit. When inserted the opposite orientation as you typically expect a diode, it begins to conduct when the voltage across it reaches the zener voltage. Then the voltage stays at that level. See http://en.wikipedia.org/wiki/Zener_diode for more information.