Poltergeist means "noisy spirit." Other ghosts can be noisy,so what is the difference? Almost everyone has a different theory of what a poltergeist is. The main difference is that poltergeist is created by the unconscious mind of a living person, usually under some kind of stress. This is why it isn't, usually, considered a ghost or a spirit.
According to some people, the poltergeist is the only paranormal phenomena that can interact with the living and move objects. I don't believe that is true, because there is intelligent ghosts that can move objects and interact with the living, and they aren't created by an unconscious mind.
Poltergeist is said to be the 2nd most dangerous paranormal phenomena. The first is the demonic ones, of course.