?what is the purpose of a zener diode generally in a circuit? cant a PN diode replace it?

A zener conducts in either direction in the right conditions.  A forward biased diode always has a fixed voltage drop across it for practical currents - 0.6-0.7V for silicon diodes, a little lower for germanium and a little higher for GaAs.  However, those voltages are fixed and can't be altered.  The zener's forward voltage drop is the same as for a regular diode, how when reverse biased its breakdown voltage can be tuned (by the manufacturer) to a wide range of values.  So, for example, if you want a diode that will always have 5.1V across it you need a zener since no diode technology naturally produces that voltage drop when forward biased.

As andrew suggested, a zener diode is just like an ordinary silicon diode up to it's zener voltage = typically 5 volts to 100 volts. It then conducts current, so that the voltage drop increases very little, until it gets so hot that it fails. With a good heat sink some have been made that can dissipate up to 50 watts. Generally you can purchase voltage regulators for about the same money with over voltage and over current protection that produce much less heat, so the high power zeners rarely find new applications. Some silicon power diodes have a sharp zener like knee at as much as 1200 volts, but typically they fail when the dissipation reaches about one watt. Usually they short out with a resistance of less than one ohm.   Neil