The "Doomsday" Book was a record of "all things" in the Realm of William the Conqueror - from chickens, to cows, to taverns and castles. Everything and anything of value was recorded in the book.  The purpose of course was to establish a basis for the collection of taxes.

So from village to village the survey went.  When a village learned that the King's Tax Collectors were on their way they would say "the day of our doom is at hand" - hence the name Doomsday (Domesday) Book.