A: A Ghost is like an "imprint" of an event, a person, an animal or a place (including objects) that are no longer in the material world.
Everything that is around has an energy and sometimes a certain event, stress, trauma or other factors can amplify that energy and it can stay around in a place for many a year even centuries. For example a person who takes the same path to work doing the same "ritualistic" moves for a long time (eg. a night watchman walking down the street closing and opening doors always at the same time) can be seen or felt walking around at the same time doing the same moves in the same area.A battle field is the most common example where centuries after confrontation battle cries can be heard, people charging and moaning just like a record replaying events as they were when the most amount of energy (no greater energy than pain and dying and passion) was released. These ghosts are not out there to make contact with the living and most of the time they are not even aware of any presence and harmless. These recordings or imprints if you wish can fade away in time and there is nothing to be alarmed of. It's like watching a mirage or other natural events that people and science more readily recognize than hauntings. Not everyone can witness such phenomena and perhaps the reason most of us are scared of it is because we know so little of them nor we were taught the difference between these and other less friendly entities. In this age of science and reasoning we can still not deny or "extinguish" other events just because we cannot explain them.( Noone told that to a ghost anyway.) Like anything these ghosts comprise of energy and taken that away or replacing it with newer or stronger ones (eg. another event at the same place) can make them fade or entirely disappear.
A spirit is however a very strong energy or spiritual or subconscious existence of a person that is passed away. Unlike the ghostly imprints these spirits don't always "attach" themselves or exist in one area only. There are times they can or trying to communicate with the living and respond accordingly. There can be many types of spirits and just like the human nature they are very complex. They could have an unfinished business, they could try to seek justice or even unaware that they are dead. One of the most common of these is the last visit of a relative or loved one at the time of or shortly after their death or regular visit of someone close at the time of need or to warn of upcoming events. However again, we strongly advice against trying to communicate with spirits as unless you know what you are doing and exactly what you are dealing with you can invite a great deal of trouble (and at times even if you know what you are dealing with)
An elemental is something more open to imagination. Every tradition and folk magic from ancient cultures to modern Animalism and Neo-Paganism has them, call them different names and summon or keep them away with a wide variety of rituals. But they all agree on one thing, they are there and only those highly trained should address them, only a handful special if that can have power over them and that they should be respected or entirely be left undisturbed. They are lesser nature spirits who not unlike humans have personalities but not a "soul" as such.( Old folktales say that some elementals are envious of humans and want to obtain a part of their life. In the case of an "incubus" they can obtain their own soul by sleeping with a mortal female and in the form of the fruit of that unnatural union, a newborn child. So that is the folk tale. )Some folklore consider elves, gnomes, mermaids elementals as well but we have nothing to worry about those on this website.
Elementals like to attach themselves to human energies and can be seen or sensed in a person's aura by those who sensitive to it.
Some are mischievous and some can cause trouble and series of bad luck or tragedies and illness. They can appear alluring at times granting the host special psychic abilities then play nasty lil tricks and mind games or frighten a human. Just like anything that feeds on energy elementals are searching for similar energies in a host or for those who are vulnerable to psychic attack.( eg. sick, very frightened, ridden with guilt or complexes, depressed, an addict, those who believe that "lil people" are sent by their enemies to harm them, though this last comment can be interpreted in two ways)
And as for demons....They are again, nature spirits with a predatory nature to live off human energies like a parasite almost always causing havoc. Many a civilisation believed that they can attack humans through the mouth, hence the origin of the tradition of covering your mouth when yawning. And that's all I have to say about THAT. And I am content