Orbs is the popular name given to (typically) circular anomalies appearing in photographs. In photography and video, orbs appear to be balls, diamonds, or smears of light with an apparent size in the image ranging from a golfball to a basketball. Orbs sometimes appear to be in motion, leaving a trail behind them.
There are two main schools of thought regarding the cause of orbs in photo images. The first school sees it as a fairly clear-cut case of flash reflection off of dust, particles, insects, or moisture droplets in the air in front of the camera, i.e. they are naturalistic. The opposing school maintains that orbs are paranormal in nature, i.e. non-naturalistic.
While some people claim that orbs are more likely to appear in certain locales, or are attracted to human actitivities (especially those involving children), the images on the internet fora devoted to orbs are taken in graveyards, backyards, attics, and kitchens, as well as bars, convention centers, and city streets -- in short, anywhere people may be taking photos.
As orb photos may demonstrably be gained anywhere, the position of "ghost hunters" who claim orb photos are more common in allegedly haunted areas is significantly weakened. That orb photos can be gained anywhere has, however, been adopted into the position of those who maintain that orbs are probes/devices being used by an alien culture to monitor human activities.