From the story
Professor Binns teaches History of Magic at Hogwarts. He is the only teacher who is also a ghost: he fell asleep in front of the staff room fire one night and rose the next morning to teach, leaving his body behind.
Discovered in Book 2, Chapter 9, The Legend of the Chamber of Secrets
Professor Binns' class is considered to be the most boring at Hogwarts. He is very knowledgeable about the history of Hogwarts and dislikes legends and fables, preferring to deal with facts. He holds the class's attention, for perhaps the first time ever, when he recounts the legend of the Chamber of Secrets.
Discovered in Book 4, Chapter 15, Beauxbatons and Durmstrang
"Professor Binns gives the fourth-year students a lot of homework, asking them to write weekly essays on the Goblin Rebellions of the eighteenth century."