There are three major categories of ADHD, established through years of research and study: the inattentive type, the hyperactive-impulsive type and the combined type. Each classification has its own signs and symptoms, and to be considered as having an attention-deficit hyperactivity disorder, a person should show at least six of the signs.

The signs and symptoms vary from one person to another, but the general indications of ADHD are: difficulty paying attention, difficulty in finishing tasks, frequently jumping from one activity to another, problems focusing and following instructions, often losing and forgetting things, being easily distracted and irritated, difficulty paying attention to particulars, trouble organizing tasks and activities, difficulty waiting, barging in on other people, blurting out answers before questions are asked completely, feeling impatient, talking too much, and having trouble working silently.

Impulsivity, hyperactivity, and inattentiveness are the chief signs of ADHD. These signs commonly appear in the early years of a child's life, usually by the age of 7. It is essential for a child to obtain a careful examination and appropriate diagnosis by a skilled health professional.

The first category is the inattentive type. People with this category of ADHD seem to be lacking in concentration, and seem careless and negligent. They can’t get interested in details, or have a tendency to make mistakes during lessons or other activities. They have difficulty with sustained attention in tasks or play activities. They have trouble listening. They find it hard to follow given directions. They have trouble organizing things. They avoid tasks that include mental effort. They tend to lose and forget things like notebooks, homework, or toys. They are preoccupied and easily diverted. And they are forgetful in daily activities and routines.

The second category is the hyperactive-impulsive type, which usually means having excessive energy. It seems that these people never tire out. They are always moving, doing something and are always messing around. They cannot focus on doing a certain job. People with this kind of ADHD keep on fidget and squirm most of the time. They find it hard to sit still or play silently. They are very talkative. They blurt out answers before understanding and hearing the complete question. They have difficulty waiting in line. They interrupt or intrude upon other people’s work.

The third category is the combined type, with symptoms of the two other. This category is the most common among the three categories for attention-deficit hyperactivity disorder.

Oftentimes as a person grows older the symptoms of ADHD turn out to be more manageable. Specialists believe that the ADHD can diminish as the person gets older, but the problems with focusing and attention often stay.