I'm quite accustomed to the idea of ghosts being created by a traumatic death. Murderers, and the murdered, for example, are attributed to haunting a place. If you're not into horror movies, reenactment shows, while silly, can do a good job of building up ghost and demon lore if you watch enough of them. You pick up a lot of trends, and its all based on real life events. This is just based around what I've absorbed over time, but its probably affected most by Christianity.
I think typically a ghost that could serve as a protagonist would just be called a ghost or spirit. Poltergeists, and up are generally not "people" anymore/at all.
I don't remember what culture the idea came from, but I do recall reading of ghosts being attached to a particular object, the way same way they might haunt a house. So, say someone dies wearing their wedding ring. That wedding ring gets sold or passed down through the family.. where it goes, the ghost goes. Useful if you want your ghost to be mobile!
Iron and salt are useful for protection from ghosts. You've probably heard of salt circles!
Buzzfeed's Unsolved Supernatural Mysteries series is a fun way to explore some haunted locations and hear some interesting stories. And personally, I find Paranormal Activity a good example of the way a haunting slowly escalates- it's nice because the movie is aces in atmosphere, but over all not too scary (I'm a pansy). It's not necessarily about ghosts, buuut I still think if you're delving into that stuff, it could be useful and inspiring. Though of course, not everything involving ghosts has to be horror!