What are ghosts? I guess that depends on whether or not you distinguish ghosts from spirits. They can be people who don't realize they are dead. They can be people who know they are dead but are stuck here for some reason. They can be a loved one who is kept close by someone's strong desire (wishing they had not died, not accepting the death, etc.). Or they may be a loved one who is choosing to stay so they can comfort and help you for a while. Those are all ghosts. Spirits can include angels from the devil (yes, I believe he is a real person, as well as those spirits that followed him and try to make us hurt each other).
I don't believe ghosts/spirits can hurt anyone in and of themselves, but I think that people can talk themselves into being scared - or believe that they can be hurt if it's a malevolent spirit. But, really, a person with a body has more power than a spirit without a body. The trick is to have more knowledge so you cannot be deceived if what you are dealing with is a bad spirit (the kind that never was born here and never will be born here).