“While spending the Christmas time of 1085 in Gloucester, William had deep speech with his counsellors and sent men all over England to each shire to find out what or how much each landholder had in land and livestock and what it was worth.”
The Anglo-Saxon Chronicle, Circa 1086
This is how the beginning of work on the Doomsday Book was documented at the time.
It was a tax register. It was in fact a very important stage in the construction of the modern state.
It was William the Conqueror who ordered the survey so that he could collect taxes that were owed to the previous King, but it is thought that the book was actually compiled by his successor William II (William Rufus or William The Red). They went across the land collecting information for their Doomsday Book. Doom in this sense and at this time did not have any of its modern meanings of danger and death, it simply referred to the day of judgement in the Bible, where all information about everyone was laid bare.
That is what the book attempted to do: to gather the information about every one. These are the earliest extensive written records of Britain and its populace and settlements.