One idea is that they are spirits of those that were once alive. Other ideas go into people from the future, extraterrestrials, or people from alternate dimensions. There are spirts that have never had an earthly form- these tend to be nasty.

By definition a haunting is a place frequented by ghosts.

Yes. Residual, Poltergeist, Actual. Residual is energy that continues to replay over and over like a record skipping on a record player. A poltergeist is often mistaken as a ghost, when in fact is it is most likely latent psychokinesis of a person going through a hormonal change. An actual haunting is where an intelligent entity can both manipulate the real world environment and has the ability to communicate with living people.

You could simply ask them to leave. If that doesn't work, you can consult a psychic who can cleanse a house or a priest if necessary. However, before just asking a ghost to leave, take the time to consult a paranormal group. There are times that the ghost has 'unfinished' business. After the business is taken care of they can rest.

There is a wide range of technical equipment used. We use equipment such as regular  digital cameras (photo and video) and full spectrum cameras. We use digital voice recorders to capture electronic Voice Phenomena(evps). Some higher end tools like EMF (Electromagnetic Field Detectors), infrared temperature probes, laser grids. Most of all we use our eyes and ears, sometimes thats the best equipment of all.

No, our theory is this, ghost hunting is kind of like fishing. You have to be in the right place at the right time. Sometimes ghosts just wanna be left alone and other time they wanna come out and play. Everything we do capture we try to debunk it or re-create. 95% of the time we find explainations of the cause and what we don't we consider possible paranormal activity.