This ghost appears most lifelike and surprisingly, is very common. The ghost of good fortune is very similar. This spirit appears before someone to point the way toward buried treasure or a secret cache. Once the goods have been discovered the ghost departs. The chores on earth are done and it's time to move on.
Vengeance is a strong motive for a ghost to continue to hang around after the final curtain.
The vengeful ghost is angry at some treachery done him. Perhaps this ghost is the victim of foul play so he continues to haunt a specific locale in hopes of letting the world in on his killer's identity. A few years ago a very famous murder case was solved in just this way. Some people believe that the vengeful ghost is the only true ghost there is. A poltergeist is a very disturbing prankster who throws things about, rattles windows, and in general raises havoc in the home it has decided to take up residence in.