According to the Attention Deficit Disorder Association
( http://www.add.org/help/faqs.html#1_ ), ADD / ADHD is is a neurobiological disorder resulting from problems in the dopamine neurotransmitter systems in the brain.
When the body does not get enough chemicals to the brain, it will try to increase activity in order to increase the chemical flow. It does this by physical movement most often. What the rest of us see as not being able to sit still is the brain’s natural reaction to help the body produce what the brain wants and needs.

It is a real issue and is not an excuse for laziness. Many people with ADD have trouble concentrating on one task for very long, or may get so caught up in one thing that all else is ignored.
For instance, if I have ADD and you ask me to simply walk from on room in my house to another room in order to pick up an object and then come back with it, I may well be distracted by something along the way and forget what I was going after, or forget to come back to you at all.

Some common symptoms according to the Centers for Disease Control ( www.cdc.org)
Six or more of the following symptoms of inattention have been present for at least 6 months to a point that is disruptive and inappropriate for developmental level:

1. Inattention
Often does not give close attention to details or makes careless mistakes in schoolwork, work, or other activities.
Often has trouble keeping attention on tasks or play activities.
Often does not seem to listen when spoken to directly.
Often does not follow instructions and fails to finish schoolwork, chores, or duties in the workplace (not due to oppositional behavior or failure to understand instructions).
Often has trouble organizing activities.
Often avoids, dislikes, or doesn’t want to do things that take a lot of mental effort for a long period of time (such as schoolwork or homework).
Often loses things needed for tasks and activities (e.g. toys, school assignments, pencils, books, or tools).
Is often easily distracted.
Is often forgetful in daily activities.

Six or more of the following symptoms of hyperactivity-impulsivity have been present for at least 6 months to an extent that is disruptive and inappropriate for developmental level:

2. Hyperactivity

Often fidgets with hands or feet or squirms in seat.
Often gets up from seat when remaining in seat is expected.
Often runs about or climbs when and where it is not appropriate (adolescents or adults may feel very restless).
Often has trouble playing or enjoying leisure activities quietly.
Is often “on the go” or often acts as if “driven by a motor”.
Often talks excessively.

3. Impulsivity

Often blurts out answers before questions have been finished.
Often has trouble waiting one’s turn.
Often interrupts or intrudes on others (e.g., butts into conversations or games).
Some symptoms that cause impairment were present before age 7 years.
Some impairment from the symptoms is present in two or more settings (e.g. at school/work and at home).

There must be clear evidence of significant impairment in social, school, or work functioning.
The symptoms do not happen only during the course of a Pervasive Developmental Disorder, Schizophrenia, or other Psychotic Disorder. The symptoms are not better accounted for by another mental disorder (e.g. Mood Disorder, Anxiety Disorder, Dissociative Disorder, or a Personality Disorder).