ADHD can be separated into three categories:

Attention Deficit OR mainly Inattentive type:
The child’s main difficulty is inattention as mentioned above. These children are easy to miss as they are not hyperactive, and may be easily dismissed as daydreamers. This type is more common in girls than boys.
Hyperactive-Impulsive Type:
The child shows hyperactive and impulsive behaviors. He may have trouble being calm and appears restless and impatient. In younger children with ADHD, the hyperactivity part seems to be more prominent.
Combined Type:
The child has a combination of inattention and hyperactivity and impulsivity. This is by far the most common form of ADHD.