Among those who believe in ghosts, there are many different opinions as to what they actually are and why they exist. According to About.com, the traditional belief is that ghosts are the spirits of dead people who have become trapped between their human life and the next plane of existence. Those who become ghosts typically suffered a tragedy or trauma in their human lives that contributed to this entrapment.
Those who believe that ghosts are dead people commonly concur that all humans are spirits encased in physical bodies. Those who suffer trauma in their human lives pass out of their physical bodies in death, but they are not aware that death has occurred and that the physical body is gone. For this reason, they remain "stuck" in the physical world while in the spirit form.
Another theory that explains why ghosts exist is that they are recordings or imprints of energy on the environment where they once existed as humans. This explanation is used to describe situations such as a Civil War soldier being seen in a window where he once stood guard or a child heard laughing in the room she once played. These visual images and sounds are always the same, leading believers to conclude that they are residual energies.