What is a ghost?
A ghost is the apparition of a deceased person, often similar in appearance to that person, and may be encountered in places she or he frequented, where he or she died, or in association with the person's former belongings. The word ghost may also refer to the spirit or soul of a deceased person, or to any spirit or demon.
When a ghost is repeatedly seen in a given location, the location is said to be haunted. A place may also be haunted if there is suspicion of paranormal activity. Paranormal investigators may come in with their equipment - including an EVP (Electronic Voice Phenomenon) reader, a heat meter (I forget what it's called) which is used to measure temperature fluxuations in a room or area, and occasionally they may bring a psychic or medium with them in an attempt to communicate with the spirits should they wish to talk to the investigators.
Types of Ghosts
Ghost - your ordinary, everyday spirit
Poltergeist - a demonic spirit or ghost that manifests itself by moving and influencing objects
Residual Haunting - spirit energy trapped in time, destined to repeat an event over and over forever
Demon - a being from the dark realm, often bent on harassment, possession, or harm