What used to be called Sensory Integration Disorder, and now is called Sensory Processing Disorder, is when one or more of the 5 senses are over or under active. Frequently the child with SPD has over sensitive hearing and you will see them in a public place with their hands over their ears. The input of what they are hearing is too much for their brain to cope with, and so they do the only thing they can, which is to put their hands over their ears and reduce the sensory input they are receiving.
The opposite is also true, there are children for whom receiving input from the 5 senses is not enough and they seek more stimulation. This is the child who wants to go higher on the swing, or climb up to the top of the nearest tree. who will go on every roller coaster ride and never have enough.
The child who has a temper tantrum in public is often doing the only thing he knows to do in order to stop the external stimulation. If the stimulation is too much and they have a temper tantrum, they are removed from the scene. It worked last time, so when the noise is too much it will work the next time too.
People rarely suffer from merely SPD. Frequently it goes along with autistic spectrum behaviors, or tourettes.
The best book I ever read on the subject is "The Out of Sync Child: Recognizing and Coping with Sensory Processing Disorder" by Carol Kranowitz and Lucy Jane Miller. This book explained the reason for the child's behavior. It wasn't that the child was misbehaving by being a bad child, but frequently to escape the situation. Then her second book came out, "The Out of Sync Child has Fun: Activities for Kids with Sensory Processing Disorder" by Carol Kranowitz. This l followed, because instead of just having a diagnosis I was able to do fun activities with my child.
At last I understood why my child would react so badly merely to a pat on the back, but also I was able to do something about it by training the child to learn to accept a pat on the back. The key was to train the child to cope with the situation for himself. When his clothes itched, he had the power to change them. When he didn't like the food, I trained him to say, "I'm sorry, I don't like it," instead of "This is disgusting!"
Understanding the child and giving the power to the child to change his environment went a long way to reducing the problem. My child is still oversensitive to many things you or I wouldn't care about, but he is maturing with the ability to cope.