Professor Cuthbert Binns is the History of Magic teacher at Hogwarts, and is the only teacher at Hogwarts that is a ghost; using this to his advantage he enters the classroom every morning by floating through the blackboard, often startling the students. The history behind Professor Binns’ unusual, yet not unheard of state, is that one night he fell asleep in front of the fire in the staff room and died of old age in his sleep. The next morning he went to teach his class not realizing he had left his body behind. It is unclear if he even realizes that he is dead yet.
His classes are known for their incredibly boring lectures on goblin rebellions and his voice (“a flat drone like an old vacuum cleaner”) often puts many students to sleep. In book two, Hermione asked Professor Binns about the Chamber of Secrets, and he appeared to be quite surprised that the whole class was paying attention to him for once, and he answered their questions until he became frustrated with the myth (he is interested in facts and facts alone).
According to a piece of paper of JKR’s (from when she was planning out the plot of book three), his first name is Cuthbert. However, this is never mentioned in the books, and many other things on the aforementioned paper were later changed, so Cuthbert may or may not be his real name.