ADHD is a neurological disorder. Scientists have found that in people with ADHD, the Cerebral Cortex, the Limbic System and the Reticulating Activating System are different as compared to the brains of people without ADHD. These differences explain the short attention spans, learning difficulties, impulsiveness and hyperactivity.
The Cerebral Cortex is located under the forehead. It has been found to have slower brain wave activity in people with ADHD. Brain imaging techniques have also found that the right frontal lobe is a bit smaller as compared to that of people without ADHD. This area controls the ability to pay attention, focus and concentrate. It controls the ability to plan, make decisions, learn and remember as well as to behave appropriately. The Cortex has "Inhibitory Mechanisms" that control impulses and inappropriate behavior.
The Limbic System is that part of the brain that sits above the brain stem (which in turn sits atop the spinal cord). It controls emotions, motivations, and one's survival instinct. The Limbic System also controls what is remembered and where in the brain memories are stored.
The Reticulating Activating System (RAS) is also located at the base of the brain, near the spinal cord. It receives information about the world and one's body through the senses. It stimulates the neurons of the Cerebral Cortex and is responsible for circadian rhythms (sleep & awake patterns), central nervous system activity, and attentiveness. A completely shut down RAS results in loss of consciousness or coma. If the RAS is underactive, the result is poor learning, memory and little self control. If the RAS is overactive, the person startles easily, talks too much and is restless and/or hyperactive.
Recommended Video on ADHD and the Brain:
Sources - Newideas.Net and ADHD.org.nz