How do the ghost projections work in the Disney's haunted mansion ride?

It’s a special effect called Pepper’s Ghost. 

they had this effect in the 1800s...they project onto a piece of glass or it could be a 1 way mirror..some light gets reflected back to you and some light goes through..making the ghosts look transparent

It’s a special effect called Pepper’s Ghost. 

there is something in the ceiling that projects onto invisible glass/plastic screen to create the effect you mean, some pretty good videos on youtube about it

They are real ghosts.