We have all seen them in photographs. The small circles that may have a trail after them. They appear to float in mid air. Some attribute them to reflection, others to something paranormal.
It is thought that orbs may occur from reflection of light off of solid particles such as dust or pollen or liquid particles such as raindrops. Due to the decrease in the distance from the lens to the flash on most small digital cameras common today, There is increased incidence of these orbs. The appear as transparent or semitransparent circles with purple fringing or other chromatic aberrations. Under water photographers may also experience these orbs for the same reasons, small sea life or floating particles near the lens of the camera.
More interesting is the paranormal explanation for orbs. They too are believed to be a reflection but instead of from small particles it is from reflection off of spirits, demons, angels, etc.
What you believe is up to you. Personally, I think it is hard to determine either way. There is not a good way to test this experimentally, as you cannot prove the existence of a spirit when you are snapping a photo. I tend to lean towards the paranormal explation however. If it were a mere reflection of light off of particles in the air, would you not see these phenomena in every photo taken. This is not the case. Too much cannot be explained to really determine what the cause for these entities could be.