There are two types, one type relies on the curve of the current against voltage of a forward biased diode.More then one diode can be used too give an extended voltage range. The other type regards the diode as "perfect" and uses different series resistors and DC bias voltages so the circuit sees a different impedance depending on which diodes are turned on by the DC bias being exceeded. The diode network is often used as part of the feedback network of an amplifier, so the diodes losses can be compensated for.
Frank