Updated December 04, 2014.

A lot of people talk about ADD and ADHD as if they were two different things, reserving ADHD or Attention Deficit Hyperactivity Disorder for those kids who are hyperactive and ADD for those aren't.

Both are simply generic terms for Attention Deficit Hyperactivity Disorder though and don't really describe the type of ADHD a person has. They are often used interchangeably by teachers, doctors, and parents, which can confuse some people.

Types of ADHD

To be clear, to describe the type of ADHD a person has, you should think about the type of ADHD symptoms they have and use the following terminology:

• ADHD, Inattentive Type, which includes those children who mostly have symptoms of inattention, such as not being able to pay attention to details, getting easily distracted, being forgetful, etc.
• ADHD, Hyperactive - Impulsive Type, which includes those children who mostly have symptoms of hyperactivity and/or impulsivity, such as fidgeting a lot, having trouble staying in his seat, talking excessively, being on the go, interrupting others, having trouble waiting for his turn, etc.
• ADHD, Combined Type, if the child has all of the major symptoms of ADHD, including symptoms of inattention, hyperactivity, and impulsivity

ADD is really just older terminology and shouldn't be used to describe a child with ADHD, Inattentive Type, at least if you really want people to know what you are talking about without having to explain yourself a little more.
People still say ADD all of the time, and likely will continue to do so, since it is easier to say than ADHD and almost seems more natural, but you should use it as a generic term, like 'my child takes medicine for ADD,' and not as a replacement for the type of ADHD that your child has.

Doing so would be like saying 'HD' for those kids with ADHD that are just hyperactive and impulsive...