Addreall, like most stimulant drugs, brings the less active areas of the brain more into alignment with the more active areas of the brain, reducing hyperactivity (when the more active areas control external behavior) and inattentiveness (when the more active areas control internal behavior). Because brain study is still in its infancy, they don't have clearly pinpointed areas/high-low brain function combinations that each drug affects, so finding the ultimate prescription that works for each patient is a hit-or-miss trial-and-error method. Unfortunately, IF the first guess or two is way off, and the side effects of improper dosage and medication scare parents, the child ends up without any medication, and in trouble in school and eventually in trouble at work as an adult. Prescribing this stuff is more an art than a science.

My son is on Adderall, and it works wonderfully for him at the proper dosage. They started him at too high a dose, he got the shakes, and his nose bled like Niagara. Now he's on 10mg, has no side effects at that dosage, and he benefits noticably from it. Without it, he's completely inattentive. With it, he has started learning to read, and can hold a conversation with us without totally spacing out halfway through.
References :

http://www.playattention.com/attention-deficit/articles/category/adhd-children/