Do ghosts or spirits exist?
Ghosts do exist but you may not be able to see them too well, depending on the circumstances of their death. Ghosts are life essence weakly held together by their will -- a result of the strength of their emotions. If the individual "ghost" is too weak or if a more powerful "Gathering" consumes it, they will join and the "Gathering" becomes bigger and thus more powerful. "Gatherings" are a body of life essences big enough to become a single, somewhat sentient being (a spirit -- no longer a human per se since it combines all the ghosts' consciousness). "Gatherings" (or spirits) can be overall bad or overall good, meaning they can be monsters or they can be like spirit guides. Also, angry ghosts are easier to notice than benevolent ghosts since they are mostly violent and deadly (they make themselves known). You rarely see a good ghost but you can definitely see a good spirit.
WHY do they exist?
The bad ghosts exist because they died a violent death or died extremely unhappy and their life essence floats around the Earth until enough are collected in one place. Once enough is collected, the life essence transforms into a vile spirit bent on revenge on humanity.
Good ghosts exist simply because of a strong desire to do more for humankind even after death. They are very altruistic and die hoping they will be of service one last time.
There are SOME unique cases when it comes to ghosts. The examples include (but are not limited to) vengeful mothers and protective mothers. Vengeful mothers, due to strong emotions, will never appear as a jumble of life essence. Rather, they will either take the body of the closest living creature (possession, usually of a predator or big-enough animal, possible only through a combination of extremely focused will through maternal instinct and strength derived from powerful feelings) and make it its mission to kill the baby's killer in this form or take revenge through haunting and torturing the killer using her own physical appearance. Protective mothers (dead but child alive) will take on a guise of a creature and watch over the child over the creature's lifetime.
Where are these spirits SUPPOSED to be? Heaven? An afterlife? Rebirth? Gone forever?
They are supposed to reincarnate after dissipating into the afterlife but good spirits can bless someone who needs life essence (sickly, dying), after which they will dissipate. Bad spirits, however, will try to kill every human they see (consuming their soul) and they will grow from the life essence consumed, refusing to dissipate into the afterlife. Humans or creatures can destroy the bad spirits and force them to start anew in the afterlife by scattering their life essence.
Can people see them?
Yes. Mere life essence (like from a badly scattered, weak ghost) will be seen as golden, floating dust BUT a Gathering will take a form of a monster or a good spirit.
Are they tangible? Can they touch things?
As Dust (weak ghosts), not really. As a Gathering, in a way, yes. With enough force they can hurt, but still not touch (think of how wind can cut you due to force and particles in the air).
Can they possess people?
Yes, but it's really hard to. The spirits may risk dissipation if they don't have enough strength or the will.
Can they come back to life?
No, unless they reincarnate (lose previous life's consciousness).
Can they be destroyed or send away?
Yes they can (explained in next question).
How would you prevent or protect yourself from them?
Take a bright source of light (like a flashlight, not fire) and flash the bad spirit until it scatters. It's harder the bigger they are. You could also charm the spirit and convince it to dissipate or just talk them to it if they're willing to listen. You can protect yourself by carrying some kind of container filled with water. Spirits are afraid of getting trapped in water.
What are some MYTHS about them that people believe that are NOT true?
One myth is that all the creatures they possess are bad (many people are deathly afraid of animals because of stories involving them killing people -- but not all creatures will go out of their way to do such things so it is very likely they were possessed.) Another myth is that a mere apparition will curse you and you will die in three days. The unlucky person will be isolated for three days as the others do not want to be jinxed (In actuality, for one to die, their soul must be "touched," but they can also be physically wounded to death, and you will die immediately -- NOT in three days. Also, you will very likely be weak, not dead, depending on strength of bad spirit.)
Are they a common thing?
Very common in forest areas since trees help keep life essence in place to gather.
How do people react when you tell them you've seen a ghost?
They will lock you away for three days, and if you survive, they look at you strange because according to their misguided beliefs you are supposed to die. Also, your survival indicates to them that you are allied with the bad spirits, so you may carry that stigma with you.