None. . These are merely diagnostic labels imposed by the dsm(diagnostic & statistical manual of amer psych acad). Current nomenclature is that everybody is "ad/hd" with some variation of intensity. Next dsm up will drop adhd label in favor oc something else. It's just a name.
No longer an issue. Because the underlying neurobiological and neurochemical basis is the same for both, what once was called add was changed to adhd-primarily inattentive type. Adhd-primarily hyperactive/impulsive type is seen in younger children. Children with adhd-combined type are more commonly referred for treatment, as the hyperactive/impulsive symptoms draw attention. See chadd.Org for full explanation.