Strange as it may seem, a true ghost is just a creature able to cause the same damage as a normal person: voice, various noises, contact with objects. It may also generate wind to shut doors or windows. The sensation of cold is often associated with the ghost.
Cranky ghosts occasionally suck energy in from their locality, creating what is known in the spook hunting community as "cold spots". "(If) you are feeling overly tired or depressed, wanting to keep your drapes and shades drawn and be in the dark, and you are feeling withdrawn and tired all the time, with no good reason, you may be having your energy being sapped by an attention craving ghost."
Defying Ohm's law of resistance, ghosts are able to maintain a distinct electromagnetic field in a defined yet apparently homeostatic area of the atmosphere.
It is said that some ghosts, called spirits, do not have the energy to manifest physically. Instead, they have developed the ability to rearrange the molecules in the local area to correspond with a structure discernable by the olifactory glands. So stop blaming the dog for that sudden gust of burrito; it may just be a friendly ghost, hoping to communicate with you.
Definitions about ghosts