According to Healthline, "ADD" stands for attention-deficit hyperactivity disorder, which is more commonly called ADHD. While ADD used to be the prevalent acronym, ADHD has officially replaced it, as of October 2014. About.com adds that some people still use ADD, particularly in cases of little or no hyperactivity.
Healthline says that symptoms of ADD and ADHD can be mild to severe. Some people become hyperactive when they are tasked with an activity they find unenjoyable. They are capable of remaining at attention for tasks they enjoy, however. Concurrent issues, such as depression, can worsen ADD and ADHD. In some cases, symptoms are less noticeable as a person gets older.