Orbs: What are They?
When a ghost hunter is investigating a haunted house, the most common anomaly that they will photograph and video is a phenomenon known as "orbs". These are small round balls of translucent light that float around when observed through a night vision device. This can either be a video camcorder which has the night shot feature built into it or a night vision scope that people can buy from an electronics shop such as Radio Shack. Some ghost hunters go as far as military night vision goggles that are out of most peoples price range, but we find these useless along with scopes, as the only thing they would be good for is personal proof. You cannot record on them and at the end of the night you only have your word, which isn’t much help or proof to anyone else.
Another way to capture orbs is with still cameras. This can be just about any picture taking camera but we find the best ones to use are the 35mm cameras with a bright flash, or a digital camera, which is our personal choice because you can spend all night taking pictures and view the actual pictures when they are taken. If nothing shows up, then its just a simple delete process. There are a few theories as to what orbs may be, and a couple of theories that you may like to consider are these:
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Orbs are the life forms of a once living human being
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Energy of some sort, displayed when a spirit is near.
It is said that for a ghost to appear to someone in a full body form takes vast amounts of energy to accomplish, and also the reason why it is "un-often" that you would see such a thing. A ghosts most comfortable form to hold is the small energy ball known as an orb. No-one has the true answer to "What an orb is" and everything you read about them is pure speculation, so that's why there are researchers such as ourselves who have seen and heard things that we cannot explain, and want to learn further about. Some people might say that an orb is just dust floating through the air, or water spots when taken with a 35mm camera etc. This is the very reason why we chose to get video camcorder footage, rather than still pictures. Its a more believable opportunity to see them on moving film rather then with pictures, and at least this way, we can see how these orbs react when pursued and to where these orbs vanish too.

Source: Aliens the Truth
Below is a theory from FUJI FILM as to what these orbs could be, whether or not you agree with there theory is a different thing... But how do we explain moving bright balls of light that we capture on camcorders ?
Floating dust particles may cause white spots appearing at different positions on pictures taken by a digital camera using the flash. Dust in front of a subject reflects the flash light and the image of the dust is captured out of focus.
Dust, snow, rain, pollen, condensation, or any small airborne particles may cause the same problem.
The photograph will exhibit the nearby dust particle to be light and big because it is out of focus, and to be round as the shape of the aperture.
The particle will appear light and big because it is out of focus; it may assume a shape similar to the aperture of the camera, usually round.
More Orb Photo`s

Taken in my livingroom using a digital camera

Taken by Brenda Butler at Rendlesham Forest - October 2004
Taken by Seth Hobday in his back garden - Dudley, West Midlands
Updated : 4/3/2005 by Lee