Their causes of death vary as does the reasons why they haunt specific places and people. Ghosts can be human or non-human. 

They may appear as Ectoplasmic Entities or in more recognizable forms.  Spirits always exist around us though we are unable to see into their realities as they vibrate as a different frequency than third dimension.  If you compare this to tuning into a television program, you cannot link to their channel as the frequency is not in your range. 

Animals are more aware of Ghosts / Spirits than humans are as they can see and hear in higher frequency than we can.  Ghosts have personalities often similar that which they had while alive. Just because a person dies, does not mean they will become instantly enlightened, and angelic.  As on the Earth plane, it takes many incarnations of clearing and healing.  Multidimensional entities have always been part of our experience in third dimension. 

Ghosts sightings have been reported in most parts of the planet - with some reports being written, photos, drawings, folk tales, even carved on stone (hieroglyphs in caves). 

Some of the most popular places they haunt are Sacred Burial Grounds, Cemeteries, Sacred/Power sites. Places where there are strong Magnetic fields, places of Religious worship, Caves.

Places where acts of Violence occurred, Taverns, Theaters, Ancient Ruins, Castles, and perhaps even in your Home.  Many people see ghosts of family members in their home.

Ghosts are not to be confused with Angels, Spirit Guides, Nature Spirits, or other Entities from different realms. At this time in the planet's history we are going through physical and spiritual changes. Earth changes cause shifts in our weather patterns and magnetics.

All of this leads to a dimensional merging that will make it easier and easier for Ghosts / Spirits and other phenomena from the other side to be experienced in our Third Dimensional reality.