I believe orbs are just refractions in light that a camera catchs. Now as for seeing orbs in person as they flash across you for that split second, I believe can be a spirit. I have heard of occasions where that has happened during an investigation but as it is limited to photos, just a wrong capture of light. Given I've taken several pictures that end up having orbs in them and as far as I know, my house is not haunted or has a ghost.