Houses can be haunted because there have been events that have taken place in these houses that cannot be explained. Usually events or actions that occur should be able to be explained using scientific explanation and theories. However, some people are skeptical. In order prove that these houses are haunted is through actually being an eye witness to the ghost or unexplained event.
Therefore, haunted houses do exist to an extent. It may not occur every time a person thinks a house is haunted, but they do exist. When a house has unusual things happen like lights coming on or going off, pictures being flung across the room and noises coming from nothing, the house may be haunted. I have never been to a haunted house that I know of.