Basically toplines with chord notations and lyrics that you 'fake' your way through.

Just going on memory here but I think the name came about during the silent movie days. When they'd show a Chaplin film or whatever, the smaller places would hire a piano player to play backup while the movie was shown.
When the movie was sent out to the movie houses, it included a fakebook for the piano player with suggestions on music to play during certain parts of the film.