Ghosts are spectral beings who are similar to spirits with the exception that they were once living humans. Like spirits, they posses color-coded spectral energy but cannot manipulate it to the extent that living humans can. According to the author, a ghost's spectral energy is often locked to one specific purpose (such as creating butterflies) which may or may not be tied to the manner in which they died. Particularly willful ghosts may be able to interact with the physical world, earning the distinction of "poltergeists", similar to spirits capable of the same feat. Ghosts are seen to have floaty "tails" instead of legs.
Some statements PJ makes to Max suggest that people with particularly violent deaths are more likely to leave ghosts after death, or at the least lengthen the time their ghost exists. Ghosts don't linger forever; they can be consumed by spirits for their spectral energy, and even if they survive they turn "white n' forgetty" and fade away after an unspecified amount of time. So far, there are no indications of what, if anything, awaits ghosts in terms of an afterlife.
According to Mr. Spender, ghosts cannot use tools because as a being of pure spectral energy, a ghost would be absorbed completely by the spirit in the tool to fuel its recovery.