The modern pop culture image of a ghost is a partially transparent pale apparition that floats. Ghosts once appeared in many different ways in different cultures, sometimes solid, sometimes only in voices, and sometimes only in the objects they moved around. But they all represented the dead whose bodies are gone the way of all flesh, but whose souls live on among us.
Throughout human history, it appears we just can't seem to let go of the dead. We long to see our loved ones again, but at the same time, the thought of spirits wandering among the living scares the daylights out of us. Ghosts are also a way to explain away the unknown things that frighten us even more just because they are unknown. Over time, belief in ghosts went from comforting because it was evidence that our souls continued after death, to terrifying, because we can't control what mischief those ghostly souls want to practice on the living. -via Laughing Squid