Orbs are probably the most disputed form of paranormal evidence as they can be created by entirely non paranormal occurrences. Dust, water, reflections, and flying insects can all create an orb like effect. To be on the safe side we generally discard all photos from an investigation if any are contaminated with the above, especially dust or water. Simply because it becomes difficult to tell what is paranormal and what isn't.
Orbs are generally not visible to the naked eye but can be captured on infrared monitors and in photos. They appear as light anomalies that are usually circular, moving against gravity and can change direction, sometimes very quickly. They can appear in different colours, shapes, and sizes.