This has been a hot topic in the clinic I work in. We provide OT, PT, and Speech Therapy to children, many of whom have sensory integration problems.
A good book to read is "The Out of Sync Child" and then "The Out of Sync Child has fun" They are written about sensory integration disorders and in intelligent language but not full of "jargon" They are written by Carol Stock Kranowitz and you can find them in Barnes and Noble.
From what I have gleaned from the Occupational Therapists I work with, Sensory Integration Disorder involves three basic systems: Tactile (touch), Vestibular (processing movement and balance) and Proprioceptive (body position, muscles and joints). The brain needs to be able to integrate these senses to allow a person to function.
Children with sensory integration dysfunction, are either over sensitve or under-sensitive to sensory stimulation. For kids who are over-sensitive, bright light, too much movement, too much noise, certain types of tastes and textures can send them into sensory overload, causing "behavior" problems. For kids who are under-sensitive, they may be clumsy, not know where they are in space, and seek stimulation.
At our clinic, the therapists work together to help the children with these difficulties get the stimulation they need, so that learning can take place. Some kids need to swing and move a lot during therapy. Other kids need weighted vests and jackets to calm them so they can focus. Some kids need to work with the flourescent lights out, etc.
Every kid with sensory integration dysfunction is different and it manifests itself in different ways. What we strive to do is really observe each child individually and analyze their behaviors to see what they are "seeking" or avoiding. For example, a girl with autism is constantly hitting. She hits people, the walls, the floors, etc. It is NOT a behavior, rather a symptom of sensory integration dysfunction in which this child is seeking proprioceptive and tactile input because she doesn't know/feel where her hands are in space. When she hits, she gets input and knows where her hands are.
This is not an official explanation, just the way it was explained to me and the way I interpreted it. If I have made any errors in my explanation or terminology, I apologize. This is all new to me as well.
For more information, get the book I mentioned above, and find a good Occupational Therapist to speak with.
Hope this helps!
Holly