Spirits and Entities
Elementals, ghosts, spirits, ET shadows, lost souls, angels, guides and Ascended Masters have all been considered in recent years as periodic guest visitors that can actually be captured in photographs to "prove" they exist. Well, maybe they do exist but your digital photos that show up with a lot of circular specks won't prove it, especially to the scientific community. It doesn't matter that scientists may not be interested in paranormal activity to begin with--just because you have images with dots does not assure the viewer that what he is seeing is not of Earthly origin.
Dust and Weather
Consensus has it--even in paranormal societies--that the most common cause of the orbs, especially in large numbers, is simply dust. Particles of dust are easily captured on digital cameras and seem to sparkle in certain types of light. Other causes are typically relative to the existing weather, such as snow, rain or fog being present when the photograph was taken.
Humidity and Light
When light hits camera lenses at certain angles or catches the edge of a lens, you can sometimes get a resultant circle of light on the digital screen or final image. Repeated circles could be reflections or refraction from the sun itself. These orbs appear to radiate and bounce in what appears to be a line. Track the line and you'll often be able to deduce that the sun was just off-screen when you shot the picture. If the humidity is significant, this could enhance dust and other particles so that they all show up as orbs in your shot.
Film vs. Digital
These orbs don't show up in all photographs. If you were to take a photograph using a digital camera and then one using a different camera, such as a 35 mm SLR with film, the results would most likely be different. The digital can pick up the dust particles and so forth more easily. On film, the large circles that show up at times are quite different. They are usually white, purple, pink or blue. Debate continues regarding what those might be, but you can try to locate a professional who has proper equipment to inspect and evaluate your photographs.