Orbs are 99.99% of the time just dust particles. Most newer digital cameras are small, and the flash is located close to the lens. When the flash goes off, it illuminates the dust particles in the air causing orbs.
A simple test to prove this: Set up your camera on a tripod and take a photo of a sofa or chair with a cushion on it. Go over the the sofa or chair and smack the cushion hard a few times then quickly take another photo. You should be picking up lots of orbs now.