Sometimes they are used interchangeably. ADD/ADHD stands for Attention Deficit Disorder/Attention Deficit Hyperactivity Disorder. Kind of like Jill/Jilly.
Since my site is called Attention Deficit Disorder, that's what I usually use. ADD can be primarily (predominantly) inattentive, primarily hyperactive, or combined type.
Primarily inattentive means that people have trouble focusing and sustaining attention.
Primarily hyperactive-impulsive means that people have difficulty staying still or quiet. They move a lot and talk a lot. Also, they are impulsive and can interrupt quite a bit.
Combined type means that they have a bit of many symptoms-some intattentive and some hyperactive/impulsive.
Here's a great site that explains it in detail. NIMH-ADD Site