Often times, a smudge from a dirty finger, a water drop, or a particle of some sort can get on the camera lens and produce what looks like orbs in photos.  The easiest way to spot a smudge on the lens is if a similar-looking orb appears in the same spot in a series of photographs taken with the same camera.  Orbs that appear in the same section of a photograph are almost always a lens smudge.  For example, if the orb appears three inches from the top and two inches from the left in more than one photo, a smudge should be suspected.  The three photos below show different smudges on a camera lens surface: