A dictionaries definition says that a ghost is the soul of a dead person, a disembodied spirit imagined as wondering among or haunting living persons.  A spiritual being.

There have also been many different classifications of ghosts.  Here are a few classifications that I found during some research.

Ectoplasm / mist -  Shows in the from of a mist or fog with a swirling type shape.  This fog or mist is usually a few feet off of the ground and can travel at will.

Funnel Ghost or vortex – This will take the form of a funnel while moving about and swirling.  The body seems to have a long narrow shape and has the same consistency as the ectoplasm with the form of the vortex.

Cold Spots – Cold spots are believed to be ghosts/spirits that are drawing energy in an attempt to manifest.  When the ghost/spirit draws on this energy it causes a sudden and very noticeable cold spot that can move at will.

Shadow Ghosts – The name pretty much is the description.  This entity appears as a moving shadow with no real consistent shape or form.

Apparition – An apparition is the visual representation of a ghost or spirit. The best known apparition is that of seeing a transparent figure wearing clothing from the era of when the individual passed on.  Some apparitions have been reported to look as though it were an actual living person, held intelligent conversations with someone then dissipated in to nothing or walked through a solid object.

It has also been reported that certain odors have been smelled prior, during and after a sighting of an apparition.

Simply put, an apparition is a ghost that you can see that is often transparent that suddenly disappears in front our eyes or walks through a solid object such as a door or wall.