Here is the definition of a poltergeist:

The term poltergeist is German for "noisy ghost." As such, it is an apparition which is causing a great deal of trouble in a house - knocking things over, breaking objects, and otherwise making a great nuisance of itself. 

Such destructive apparitions usually only focus on material objects - they do not harm people. They also tend to occur in the company of young children or adolescents. When the children move, the poltergeist moves as well. Many attribute this to hormonal shifts in adolescents, sighting that these hormone shifts generate more energy and the 'ghost' uses that extra added energy to move things about. 

There are only three recorded incidents of poltergeist activity where investigators could show no reasonable or logical explanation for the events taking place. There have been many reports of poltergeist activities but most of the time investigations have shown that a physical human was involved in much of the activity taking place.

Hope that answers your question. Thanks for writing Eeeek!