POK is the part of Kashmir that is controlled by Pakistan which it annexed militarily against the wishes of the then ruler, Hari Singh of Kashmir. The story is a bit complicated.
During the time of Independence there were several princely states who had the choice to either merge with India or with Pakistan or stay independent. Kashmir was one of them.Hari Singh faced this situation and decided to merge neither with India nor with Pakistan. India accepted this (atleast at that time). However Pakistan had a different plan. It wanted Kashmir any how in whichever way possible. So Pakistan sent it’s army to ‘capture’ Kashmir. The military of Kashmir was definitely not strong enough to counter Pakistani Army, so Hari Singh asked India for help. India put condition ‘India will help Kashmir only if it becomes a part of India’ Hari Singh readily bought this as Kashmiris were being killed and tortured by Pakistani Army.The Instrument of Accession was executed by Maharaja Hari Singh on 26 October 1947. By executing this document under the provisions of the Indian Independence Act 1947, Maharaja Hari Singh agreed to accede to the Dominion of India.
Indian army was successful in pushing Pakistanis but before our army could completely defeat them and seeing a lot of bloodshed, the then Prime Minister of India Pandit Jawahar Lal Nehru decided to take the matter to UN. UN simply declared ceasefire. The cease fire line is known as LOC till today.The war ensued ended in retaining most of Kashmir and Jammu, Ladak .PoK and Gilgit-baltisthan was retained by Pakistan. If India had pursued the war it could have driven Pakistan from these places too.
Thus the part of Kashmir that remained with Pakistan after ceasefire is known as Pakistan Occupied Kashmir (POK).
- Map source: BBC Modified by us
- Info source: Derived from various answer given at Quora