In a nutshell, an orb is a sphere. Spheres are not necessarily ghosts. Too often people are panning for fool's gold and think that every sphere they see is a ghost. As investigators, we know better. Often, dust, a reflective surface, a camera flash, a bug, smoke and pollen are thought to be ghostly apparitions, when they aren't. Here are some wayhs to determine ghostly orbs from other things.
1.) Consider your location. Is it a dusty basement? Were you outside after a good rain? Were bugs biting your neck? Were other investigators snapping pictures? Were there flashlights? Or other equipment that produces small lights?
2.) Carefully examine your photograph- Orbs that look transparent are dust.
Orbs that seem to have faces inside them are usually moisture. The lines of the face are actually the crystallization of the water.
Spheres with tails or that have a solid center and get softer around the edges...need a closer look. Usually really solid balls with no known light source and density are probably the closest to an apparition.
3.) What is the history of the location? If you have been told that someone was killed where you are investigating, and you ask a question, or are taking base readings and photographs and you capture an orb. It could be an apparition that you catch in one of your pictures. The picture is strong evidence. When taking pictures, try not to use your flash. It can reflect off of other surfaces or instruments and create a false orb.
4.) Ghostly Mists - are the most natural, unnatural objects you will photograph. If you could capture a mist in a picture, it is either a ghost or the house is on fire. If you are outside, it could be fog or an actual mist. Be sure to document the temperature where you are and write down any events that may have happened in the hours before you started taking pictures. Note if you see a face in the mist, its changing density and if it is following you or another investigator.
5.) Rods - are streaks of light found in your photograph. These are usually objects caught in flight or are due to over-exposure.
6.) Figures and Shadows - and possible full apparitions are hitting the paranormal jackpot! Carefully examine the picture and check to see if anything could have caused such an anomaly. Debunk if possible.
7.) Glowing lights - these are more solid and usually result from bad photography and flashes. Always try to recreate the light. This makes it easier to dismiss but harder to explain when your can't.
Be aware of your surroundings. Document everything. Be skeptical. Take several pictures in the same area. Debunk or recreate light sources whenever possible. You are in informed investigator!