Who owns the land?
William the Conqueror said that all the land in England belonged to him. He kept some of it for himself and he gave some to his lords. They became William's barons (another word for lords). The most important barons were called earls.
The barons and earls were the leaders of the king's army. They had helped him to conquer England, and now they helped him to rule it. The chief men in the church - the bishops and abbots - were just as important as the barons and earls. They too were given land by the king.
Each baron, earl, bishop and abbot paid rent for his land, not with money, but with help in time of war. He promised to lead his knights into battle on the king's side whenever there was a war. The more land he had, the more knights he had to bring. We call this the feudal system.
The lords and bishops who paid their rent to the king are known as tenants-in-chief. When they got their land, they had to kneel before their king and promise to serve him. This was called doing homage to the king. A man who did homage to the king was the king's vassal.
William did not always trust his great lords. He knew that if the king was not strong, they might rebel. So in 1086 he decided to find out how much land each of them had. He sent his men to each county in England. They had to find out who owned the land, and how much it was worth. They wrote their report in what we call Domesday Book.
Walter Robson: Medieval Britain; Oxford University Press, 1991/2000, page 6