Like their name implies, a race of spirits that dwell inside of bodies already turned into skeletons and makes them move. As Elizabeth says, they are like hermit crabs. Their new bodies only consist on raw bones with no flesh or muscles covering them, so the movements are more slower and erratic than other beings.
As the souls of the deceased appear when the physical body ceases to function and the spiritual form separates, some of them try to go back only to find that their flesh already descomposed and returned to the soil long ago, so they let this mischevious spirits enter in their bones and become their dwellers, using them as husks. Of course, given that to think that the one inhabiting the skeleton its the original owner is proven wrong very commonly. Typically, people don’t allow their bodies to be used, but it seems that many people allow spirits to do so in England and generally Europe, land of the spirits.