I don't believe in them in the way that most people do.

I don't believe that they are the "spiritis" of the dead, but have 2 theories:

(i) They are physical echos, appearing through time, maybe through some sort of time warp or something.

(ii) They are people from another dimension (ala Sliders - many dimensions on Earth, all following different event paths), and a rift between the dimensions allows us to see them for a brief time.

These time warps or rifts or whatever they are called really, they may be stronger in some places, which is why some places seem "Haunted".