ghosts and poltergeists

[Takappalt]

I was just wondering if anyone knew the difference in the definitions of "poltergeist" vs. "spirit" vs. "ghost" and so on. I was told by a friend that a poltergeist is the only thing of that nature that can actually physically harm people (or just interact physically) and that ghosts and spirits can not. But due to recent events I have to disagree with her. She also told me that ghosts are beings that have always been around, and that spirits are the souls of dead people that have stayed behind after people die. Can anyone give me their opinion / ideas on this? It would be much appreciated. Thanks.