It's not either technically. From what i know, it's a sign of a ghost. When I went to Gettysburg, the tour guide for a ghost walk was explaining how if you snap some pictures with orbs in it, it can be really exciting. It you zoom in and see close enough you can see different patterns, or even faces of people if it's legit.
Most "orbs" seen in photos are dust... A dirty camera lens or dust getting caught by the flash of the camera are both perfectly reasonable answers to orbs. I wouldn't call dust good or bad, annoying may be a better term for it.
orbs that are captured on camera are just dust particles reflecting light. ghost/orbs/spirits do not exist.