Cuthbert Binns is the History of Magic teacher at Hogwarts. He is, in fact, the ghost of a Hogwarts teacher who died in his chair one day, but apparently did not notice that he had shuffled off this mortal coil and carried on with his duties. The students find his droning lectures dreadfully dull; in fact, only Hermione Granger seems able to remain sufficiently alert in his classes to take reasonable notes.
Binns hasn't appeared in the films, or the PC games. His explanation in Harry Potter and the Chamber of Secrets of the legend of the Chamber of Secrets is instead given by Professor McGonagall in the film, in the PC game it is Professor Flitwick who tells the legend.
He does not know any of his students names, and ironically does not even recognize Harry Potter, thinking him to be Perkins, even though he teaches History of Magic.
All three of the main characters (Harry Potter, Hermione Granger, and Ron Weasley) were in Professor Binns' class for a number of years. His lack of appearance in any of the first five movies argues for the fact that he will never be portrayed in the movies.