Sensory processing refers to the way the nervous system receives messages from the senses and turns them into responses.  For those with Sensory Processing Disorder, sensory information goes into the brain but does not get organized into appropriate responses.  Those with SPD perceive and/or respond to sensory information differently than most other people.  Unlike people who have impaired sight or hearing, those with Sensory Processing Disorder do detect the sensory information; however, the sensory information gets "mixed up in their brain and therefore the responses are inappropriate in the context in which they find themselves.
 
A more formal definition is: SPD is a neurophysiologic condition in which sensory input either from the environment or from one's body is poorly detected, modulated, or interpreted and/or to which atypical responses are observed.  Pioneering occupational therapist and psychologist A. Jean Ayres, Ph.D likened SPD to a neurological "traffic jam" that prevents certain parts of the brain from receiving the information needed to interpret sensory information correctly.