When a pure semiconductor is doped with the impurities of elements like Galium or Aluminium, it becomes p-type semiconductor. A p-type semiconductor is an extrinsic semiconductor which has holes as the majorities charge carriers.
A Zener diode is simply a diode (i.e. p-n junction) which is operated in reverse biased condition and that too in the breakdown region.
A Zener diode is used as voltage regulator in the electrical circuit.