I'm of the opinion that most (and note I say most) orbs are nothing more than some sort of airborne particulate. When I did my first investigation in the summer of 2000, it was of an abandoned cemetery in Madera. There were five of us walking around stirring up dirt and then taking pictures using the flash. Of course dust orbs were caught on film, but we didn't know better at the time and got so excited on how we were capturing so many ghosts (we postulated that since it was a cemetery that explained why there were so many ghosts). Only years later did I realize that these were dust orbs and felt rather stupid about it for believing they were something supernatural.
I receive some e-mails from time to time with a photo attached with the person asking me if they caught an orb. Usually they are easily explained as a dust orb. But when I respond to the sender, they usually are disappointed and sometimes even upset that I have determined their photo to be non paranormal in nature. Unfortunately it seems that some think that orbs are proof of ghosts exist given all the websites and groups out there that still think this to be the truth.
I've always tried to show people that orbs are nothing special, stating how even in a recently vacummed room there can still be dust in the air. I've always said that if anyone wants to see ghosts for them to smack the cushion of their couch once or twice and then snap a few photos with the flash on and they'll see what a dust orb looks like.
I found this video done by South Coast Paranormal, who is in Southern California, that is an excellent example of what a dust orb looks like.
As you can see, the person in the video hits the cushion several times and dust flies up and moves around the room. I'm of the belief that anyone who is interested in ghost hunting, or just wants to take photos of a haunted place view this video first. Though I doubt that this would end the debate on orbs as there are some groups and people out there that want evidence and to believe to bad that anything to them could be considered paranormal. Though hopefully with a bit of education that will change.