2 possible explanations for orbs:
(and bear in mind, this is coming from an open-minded skeptic!)
1) They're some form of paranormal energy, invisible to the naked eye, but not to camera or video equipment.
2) They're dust particles. They are illuminated either by the flash of the camera, or the ambient light of the surrounding area. The number which show up on any given frame depends on their proximity to the camera lens at the instant it captures an image. We often fail to realise quite how much dust is in the air - it's only when we see a stream of light coming through a window that we notice how much dust is airborne. This effect is less exaggerated at night, since temperature (and thus, air currents) are less, and dust tends to settle more. Unless disturbed by, say, well-meaning ghost-hunters.
What I find interesting about orbs is not so much their appearance on camera film - which can be attributed to many causes - but their appearance on video equipment. They do, in some cases, seem to respond to requests from legitimate researchers, in a way that would seem to suggest some form of intelligent consiousness. They appear, move, and even flash, in a way quite unlike a dust particle or insect does. Many people dismiss this evidence out of hand since they find the very idea of ghosts, or the paranormal, or life after death, ridiculous, or against their scientific beliefs. Likewise, many people believe in the paranormal absolutely, without really questioning what else could be behind things such as orbs.
As I say, I'm an open-minded skeptic. Until I experience something in person, which I cannot explain by any rational logic, I'll have to be content with the knowledge that there MAY be parts of this world beyond conventional reasoning.