Typically, the key behaviors of ADHD are inattention, hyperactivity, and impulsivity. However, because these are symptoms which are more qualitatively observed, many have mentioned the difficulty in drawing the line at where normal levels of inattention, hyperactivity, and impulsivity end and "clinically significant levels requiring intervention" begin (Guyer)

According to Google Health, diagnosis requires that symptoms be observed in at least two different settings for six months or more and to a degree that is greater than other children of the same age.

National Institute of Mental Health(NIMH) divided ADHD symptoms into three predominant types: Predominantly Inattentive, Predominantly Hyperactive-Impulsive, and Combined. The symptoms for each type are as follows (and also available here):

Predominantly Inattentive

• Be easily distracted, miss details, forget things, and frequently switch from one activity to another
• Have difficulty focusing on one thing
• Become bored with a task after only a few minutes, unless doing something enjoyable
• Have difficulty focusing attention on organizing and completing a task or learning something new
• Have trouble completing or turning in homework assignments, often losing things (e.g., pencils, toys, assignments) needed to complete tasks or activities
• Not seem to listen when spoken to
• Daydream, become easily confused, and move slowly
• Have difficulty processing information as quickly and accurately as others
• Struggle to follow instructions.

Predominantly Hyperactive

• Fidget and squirm in their seats
• Talk nonstop
• Dash around, touching or playing with anything and everything in sight
• Have trouble sitting still during dinner, school, and story time
• Be constantly in motion
• Have difficulty doing quiet tasks or activities.

and also these characteristics of impulsitivity

• Be very impatient
• Blurt out inappropriate comments, show their emotions without restraint, and act without regard for consequences
• Have difficulty waiting for things they want or waiting their turns in games

It must be noted that almost all people exhibit some of these behaviors to some degree, but generally not to the degree where the behaviors begin to interfere significantly with a person's work, relationships, or studies. Note: These characteristics of ADHD have been identified to be consistent even across different different cultural contexts.

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References

• "Attention Deficit Hyperactivity Disorder (ADHD)." Google Health. Web. 02 July 2010. .
• "NIMH · Attention Deficit Hyperactivity Disorder (ADHD)." NIMH · Home. Web. 02 July 2010.
• Guyer, Barbara P. ADHD: Achieving Success in School and in Life. Boston: Allyn & Bacon, 2000. Print.