As defined on Wikipedia, Spirit photography is a type of photography whose primary attempt is to capture images of ghosts and other spiritual entities, especially in ghost hunting. It was first used by William H. Mumler in the 1860s. Mumler discovered the technique by accident, after he discovered a second person in a photograph he took of himself, which he found was actually a double exposure. Seeing there was a market for it, Mumler started working as a medium, taking people's pictures and doctoring the negatives to add lost loved ones into them (mostly using other photographs as basis). Mumler's fraud was discovered after he put identifiable living Boston residents in the photos as spirits.
Some paranormalists believe that there are true orbs among the misidentified ones (typically, there own orb photos) and that they can distinguish the two easily. Others say that orbs are not spirits at all, but energy being transfered from batteries and power-lines to the ghost, allowing it to manifest. This idea is easily shot down by the fact that energy travels in the path of least resistance. Traveling down a conductive wire and then exiting through an insulated coating then traveling through the air in a globular form truly breaks the laws of physics. Sorry.