The word words comes from the German tern poltern - "to knock" and geist - "spirit". This phenomenon has been researched extensively for years. It has been the subject of movies, but I don't think Carol Ann and her family were being plague by a poltergeist in the movie. There are a few different theories on what poltergeists are. I am going to discuss the one theory that I believe here and keep it simple as possible.
A typical poltergeist haunting seems to always consist of noises, such as loud knocking, and objects moving about on their own. There can also be voices, smells and apparitions in addition to the other symptoms. Usually these events are witnessed by numerous people in the home , including visitors. The events may start slowly with small noises and small objects moving and then may progress to something like furniture flying around the room and full bodied apparitions. The exact events differ from case to case as well as the progression and severity. Normally there is one person who seems more affected than others and is always present in the home when events occur. When that person is removed from the home, the events cease. This person is usually a preteen female who's hormones and the stress of that age combine and finally release subconsciously in a way that causes the poltergeist activity. The person is causing all of the phenomenon with the subconscious mind and they are not aware that they are the cause. This person can also be a teenager or into their early twenties and may be under some unusual amount of stress. When they are relived of the stress, sometimes by moving from the location, the problems stop. If the stress or causes continue, the "haunting" seems to follow them. Reliving the inner problems is the key to stopping the poltergeist activity.
Poltergeists are not often easily discernable from other types of spirit hauntings. The symptoms are often very similar. An onsite investigation and extensive research is often necessary to uncover the hidden existence of this type of phenomenon. This explains why many questions that investigators ask seem off base when they interview witnesses in a home. They are trying to rule out a poltergeist problem before they start the investigation into the possible spirit activity.