ADHD and ADD (Attention Deficit Disorder) are closely related conditions, but they are not exactly the same thing.
In the past, ADHD was divided into three subtypes: predominantly inattentive, predominantly hyperactive-impulsive, and combined type. ADD referred specifically to the predominantly inattentive subtype of ADHD.
However, in the most recent version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), published in 2013, the subtypes of ADHD were collapsed into two categories: inattentive presentation and hyperactive-impulsive presentation.
So, while ADD is no longer a separate diagnosis, some people still use the term to describe the inattentive subtype of ADHD.
In summary, ADHD and ADD are related, but ADHD is now the preferred term that encompasses both the inattentive and hyperactive-impulsive presentations.