When we die, by definition, our soul and our subtle bodies begin an ascent to higher dimensions (the etheric and mental planes of Earth), leaving our “solid” plane and returning “home” where another stage of learning, rest, preparation for another incarnation, etc, begins (time is simultaneous, not linear, and all our incarnations occur at the same time, however, it is the best way to “describe” it as it is difficult to perceive it and understand from that perspective). The route to these other “planes” is accompanied by guides or beings who work at the accompaniment of souls of the people who have just passed away, and they always meet all those who are in process of transition, being our own guides the main ones in charge of it.
However, many people die in shock, in panic, in extreme situations, or simply they are so afraid of death that when they die, they do not want to or cannot accept that they have to leave, and literally ignore the call of these beings who come to assist them. These souls become what we call “wandering” or discarnate spirits, and they stay, almost in all cases, close to a familiar or to a loved one who serves as a source of energy to survive as an “entity”. These spirits may realize they have died but they refuse to leave this plane so with the permission or not of those who serve of an anchor to them, they remain in the lower planes of the structure of our planet, where they survive from the energy of the aura and from the electromagnetic fields of the person they are hooked to.
When a discarnate is still tied to our dimensional plane, he begins to learn “tricks” to obtain energy without depending on the energy of the person he uses as a “source”. These souls maybe lost their physical body tens of years ago and have learned to survive from the energy of the environment, from the collective unconscious of the family, or from several people at once, etc. Since they have some control and power over their source of food, they really don’t depend of a single person so much and they can move at ease. In any case, they don’t have to cause any kind of trouble, they’re just out there wandering because they still feel anchored to their previous life on Earth, and don’t want to see or can’t find the exit. As they can become dense enough to be seen partially by humans, then we tend to name this kind of beings “ghosts”.
A poltergeist is something more serious, as they are ghosts, entities or beings that have been for a long time (even hundreds of years) discarnated or transiting in our physical or etheric dimensional plane, who have mastered the subject of energy, live quietly from the environment and can consciously manipulate it, moving objects, making noises, etc. Most of them are not harmful, but they look for recognition and look for help from people to get out of the situation where they are because they can’t find the way to “cross to the other side”.
A poltergeist is like a five-year-old boy doing tantrums and with the same temper when we ignore him, but obviously with a bigger chaotic way of express it. Poltergeist are usually anchored to a place not to a person, so leaving the place where it is makes you avoid this “little games”. To help it get out of this plane and go back to his “home”, we obviously need a big cleaning work of energy where he is anchored. It is also possible that poltergeist phenomena is not related to all previously mentioned but some living person with latent potential paranormal could manifest similar situations or even negative entities can create the same situations, all of it is possible within this classification. So, in general, the “poltergeist” phenomena can be due to a wandering spirit or soul looking for help, a negative entity looking to frighten us or living people unaware of their physic abilities and provoking the same situations we all have seen in more than one horror movie.