In 1085, Duke William of Normandy had been ruling England as King William for twenty years (it all started here). He decided it was finally time to take inventory of his property. He sent his agents (3-4 commissioners for each of 7 areas the country was divided into) to make a survey of everything south of the border with Scotland.* They met with groups of representatives (barons and villagers) and asked a series of standard questions. By the end of summer in 1086, the reports (in Latin) were all being compiled back at Winchester, along with data on the value of the land and its assets immediately pre- and post-Conquest. The entire work is in the same very neat handwriting, so a single scribe was given the job of compiling/collating everything. The official name of the result was "The Book of Winchester."
So why is it usually called "Domesday Book" now? That nickname was given to it about 100 years later, and just like it looks, it means "Doomsday." The idea behind the nickname is that the book was such a complete listing of everything in England that it was equivalent to the "Book of Life" used by God at the end of time to judge your deeds. It was that complete.
Except, of course, it wasn't.
For one thing, work ceased by the time King William died in September 1087. The section on East Anglia hadn't been compiled into the total work. There is, therefore, a "Little Domesday Book" with the East Anglia data. Also, important cities such as London and Winchester were not included, probably because William figured he knew them well enough and didn't need an accurate accounting of their property assets, such as he would want for the countryside.
Also, attempts to judge population using Domesday fall short of expectations. Although farms and buildings were counted, only heads of households were included in the population count. In castles, the number of men were counted, but the population in monasteries and convents was not. Best guesses, extrapolating from what data are included, is a population of 1.25-2 million, a far cry from the estimated 4 million during the Roman occupation.
Still, the Book contains a lot of fascinating information in its 413 pages, which I will draw on in the future.
*N.B.: The border with Scotland was much farther south than it is now.