Domesday is our most famous and earliest surviving public record. It is a highly detailed survey and valuation of all the land held by the King and his chief tenants, along with all the resources that went with the land in late 11th century England. The survey was a massive enterprise, and the record of that survey, Domesday Book, was a remarkable achievement. There is nothing like it in England until the censuses of the 19th century.

In 1066 William Duke of Normandy defeated the Anglo-Saxon King, Harold II, at the Battle of Hastings and became King of England. In 1085 England was again threatened with invasion, this time from Denmark. William had to pay for the mercenary army he hired to defend his kingdom. To do this he needed to know what financial and military resources were available to him.

At Christmas 1085 he commissioned a survey to discover the resources and taxable values of all the boroughs and manors in England. He wanted to discover who owned what, how much it was worth and how much was owed to him as King in tax, rents, and military service. A reassessment of the tax known as the geld took place at about the same time as Domesday and still survives for the south west. But Domesday is much more than just a tax record. It also records which manors belonged to which estates and gives the identities of the King’s tenants-in-chief who owed him military service in the form of knights to fight in his army. 

The King was essentially interested in tracing, recording and recovering his royal rights and revenues which he wished to maximise. It was also in the interests of his chief barons to co-operate in the survey since it set on permanent record the tenurial gains they had made since 1066.
Recent research has suggested that William only commissioned the survey (descriptio) in 1085 and never intended the results to be written up into a book. Some say that it was his son and successor, William Rufus, who ordered the production of Domesday Book itself. Whatever the case, the Conqueror threw the full weight of his administrative machinery into the initial survey.

From: http://www.nationalarchives.gov.uk/domesday/discover-domesday/