There are several theories about orbs, and their origins.  One such theory is that they are simply an easier form for spirits to manifest as.  This would explain why so many orbs are seen so often, but full body apparitions are so rare.  Because the full body apparition requires more energy, and are not used as often.

     Another popular theory (and the theory that this investigation teams believes is the most correct), is that the balls of light are condensed balls of electromagnetic energy.  For example, look at a street light at night.  There appears to be a glow around the main light that is not being radiated to the street.  Watch a microwave (although not for very long-they can damage eyesight if stared at for long periods of time) while it is running, there is often a soft glow just around the perimeter of the viewing glass.  When spirits manifest, and also when orbs are most prevelant in photographs, there is a sudden spike in electromagnetic energy.  If this energy was captured on film, the most natural shape for it to take would be a ball.  The various colors represent different strengths of energy (red being the lowest, blue being the highest, and white representing the average range [all colors represented equally]).

        One final theory is that these orbs are literally spirits themselves.  Not human souls who have left their physical bodies after death, but actual spirits who have always been in the spirit world, and will always be in the spirit world, but for one reason or another have decided to allow themselves to be captured on photographic film.