Ghosts are the disembodied souls or spirits of deceased people that are still trapped in the living world, and haven't yet moved on to the afterlife. It is unknown if they truly exist.
When Princess Bean first met Luci, she mistook him for a ghost, to which he replied, "I'm not a ghost, ghosts are losers that got murdered." This seems to confirm the common pop-cultural idea of ghosts being restless spirits of the dead.
Despite this, it has been stated that souls cannot exist in the living world without a body.