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The term orb describes unexpected, typically circular artifacts in photographs. Sometimes the artifact leaves a trail, indicating motion.
The technical photographic term for the occurrence of orbs, especially pronounced in modern ultra-compact cameras, is backscatter, orb backscatter or near-camera reflection.
Due to the size limitations of the modern compact and ultra-compact cameras, especially digital cameras, the distance between the lens and the built-in flash has decreased, thereby decreasing the angle of light reflection to the lens and increasing the likelihood of light reflection off normally sub-visible particles. Hence, the orb artifact is commonplace with small digital or film camera photographs.
The orb artifact can result from reflection of light off solid particles (e.g., dust, pollen), liquid particles (water droplets - especially rain) or other foreign material within the camera lens

http://en.wikipedia.org/wiki/Orb_(photographic)