An orb is loosely defined as "the soul of a once living person or animal that appears in photographs as a spherical[ly] shaped light." Both ghosts and spirits can manifest as an orb (The Orb Controversy, Miller's Paranormal Research, 2001). Many paranormal investigators pooh-pooh most orbs in photographs as dust, small flying insects, light refractions, et.al. Whatever. All I know is that we, at Ghost Hunters of Asheville Tours, get a lot of them.
An orb is the lowest-energy manifestation of a ghost or spirit. Ghosts need energy to manifest and move, but cannot create energy -- only steal it. Common sources of such paranormal theft on our tour include lightning, electrical wiring, cell phone batteries, even us! Once ghosts' reservoirs are full, they tend to be conservative with their supply so most often choose to manifest as orbs since these take the least amount of their energy.
If you get an orb in a photograph, you might want to zoom in on it as faces and words can sometimes be embedded in them.
A last, sobering thought regarding orbs: Guests reveal that the most orbs they've ever gotten in photographs are at Ground Zero.