The child suffers from mood swings, has unexplained good and bad days, can be volatile. There are many completely normal children who have some of these behaviours. If these behaviours are causing a child to significantly underachieve at school and underbehave at home, they must be taken seriously. Two things are for certain. Secondly it is caused by a minor difference in the fine tuning of the normal brain.
Heredity is obvious, as so many families have a parent or close relative who has similar problems.
For years it has been presumed that there was a slight difference in the functioning of the ADHD child's brain. Now this can be shown by the most modern research scans. Not only do these scans show a difference in ADHD, the difference can be largely reversed after administering stimulant medication. There are links to numerous sites, where you will find a wealth of information.