William Rufus was King William II of England. He was one of the sons of William the Conqueror. On the death of their father William inherited England, his older brother Robert inherited Normandy, and their younger brother Henry was given some money.
William was nicknamed "Rufus" because of his complexion, given that he was apparantly an angry young man he was probably constantly red in the face. He was killed in a hunting accident in the New Forest, and a lot of people were probably very relieved.