There is an ongoing debate between various ghost hunting and paranormal groups about what exactly orbs are.

For anyone who doesn't even know what I'm talking about....ORBS are the anomalous roughly spherical shapes captured in some pictures.

With the advent of digital photography, there was an increased incidence of these orbs being captured in the pictures.

For a time, people vehemently believed that they were capturing an actual spiritual phenomenon, that orbs were the energy bodies of those who have died and still existed on a plane of existence close to our own physical one.

With further research, however, it was revealled that many of these orbs were merely dust floating in the air, sometimes they were bugs such as gnats and moths that were flying around, other times it was moisture in the air, especially if the picture was taken while it was snowing or a light rain was falling.

Because of that, many ghost hunters/investigators have chosen to dismiss any orb pictures out of hand, assuming from the start that every and all orbs captured in pictures were easily explainable as being of physical origin.

But the fact remains that there ARE pictures of orbs that have not been able to be explained away as having a physical origin.

These types of orbs tend to be self illuminating, they don't zip around like an insect does, they also tend to be larger in size than naturally explainable orbs, and many times, these orbs are also seen with the naked eye BEFORE the picture is taken, with that orb actually coming out of and/or going into a wall or other solid structure.

There are also instances when other paranormal activity takes place, and there is a noticeable corresponding increase in the number of orbs being seen on video cameras recording the investigation. As the paranormal activity decreases, so does the number of orbs being seen.

For anyone who is taking pictures on a ghost hunt, make sure you note the weather conditions and other possible sources that may cause orbs to appear in your pictures.  The majority of orb pictures you capture will, indeed, be of a natural, physical origin.

But you may be one of the lucky ones who captures a true paranormal orb in one of your pictures.

UPDATE : 

Since my full time occupation is as a photographer, there is a point I do wish to make regarding orbs, something that explains the appearance of orbs in many more photos taken by digital cameras.

It's all about the lighting.

Today's digital camera has a flash that is located directly above the lens.  This provides a hard light that illuminates the target from one angle directly in front of the camera.

In professional photography, there is usually a lightbox or umbrella setup, to diffuse the light, and also there may be backlighting. This makes sure the lighting is even.

When you take a picture with your digital camera, it is focused at a distance beyond whatever stray dust or insects may be between the camera and the target image, but the flash hits everything between the camera and the target with a hard, undiffused light. Since the light is not diffused, and the dust, insect, etc., is only being hit with hard light from one angle, the camera cannot focus on both the distant target and the interfering particle closer to the lens.

This wasn't a problem with the old film cameras, because the flash attachment was located away from the camera body and the lens. Therefore the area between the camera and the target of the photo was hit at a different angle from the direct line of sight of the camera lens.

This doesn't mean that all orbs are thereby excluded from being of a paranormal origin...as stated before, there have been captures of anomalous images that appear different in characteristic than 'trash' orbs and are photographed at the same time as other events take place.

But in the interest of honesty and seeking the truth, it is important that we work towards eliminating trash orbs, while seeking to find any that would fall into the category of unexplained phenomenon.