The Photos
First and foremost ... the paranormal is a field that is yet to be proven one way or the other. All theories about photos, EVP, etc is simply theory. So with that in mind ...
When going through your photos, you really need to think about each thing as you see it. Was there walking a second before you shot the image? Was it windy, humid, dry, cold, hot, etc. Keep an eye on every corner and tiny bit of each image with questions in your mind as to how the 'orb' or 'mist' got there. As with anything in this field, you need to carefully eliminate the natural causes of what it might be before you can call it a true paranormal photo.
Also, remember one very important thing about photography in this field of study ... you might shoot off 300 images, and only one could have something on it. You could shoot off 20 images and wind up with 10 orb photos. You could fill 50 memory sticks and wind up with nothing. So if you walk away with nothing, don't be discouraged. It just wasn't your day, but maybe the next time you try, it will be. No matter what, keep your mind clear of all bias and expect nothing. If you THINK you'll have photo's filled with orbs, you will want it so much you may mistakenly identify dust as orbs.
Now, onto the categories!
These are the most commonly photographed, and most commonly mistaken. An actual orb is thought to be a form of supernatural energy. Usually round and bright, they show up on images as round floaters.
ORBS
You take a roll of film, and they all have hundreds of orbs everywhere! Wow! You're the bomb!! Right? Nope. Around here, we call these kinds of images pea soup ... you want to watch the air in front of your camera as you shoot the image. If you see dust in the flash, then chances are that image will be covered in 'orbs'. Note the image number, wait a few moments, and then take another image. Natural causes for 'orbs' are everywhere, and you've got to be really careful to know if these are around. Natural reflective surfaces, when the flash hits them will show up as orbs of light on the edges of the flash reflection ... these are simply not real ghostly orbs. Also massive humidity, or sprinkling rain or snowfall can often be seen as orbs in your flash. Mistaken orbs are everywhere on the Internet.
Other causes for false orbs that we don't yet have images for include bugs, rain, reflective surfaces, and dry weather can produce 'fairy lights' which will look like tiny, tiny bright dots.
Here are some natural causes examples to watch out for: