Adderall is amphetamine. It causes wakefullness and enhanced concentration, and is used to treat ADD and more generally, fatigue. Like many drugs, it was originally developed by the U.S. Air Force for pilots.

I take adderall. The psychiatrist thinks I have a sleep disorder, but really I just have bad work habits and an immense courseload and barely sleep. In addition to waking me up in the morning and keeping me awake through class, it helps me concentrate on the work I have to do. Put another way: the first time I took the SATs I wasn't using adderall, the second time I was. In between I studied only a very slight amount. My score went up 70 points.

That is, adderall is a performance drug.

UPDATE: I no longer take adderall myself, but have discovered that it makes wonderful currency in terms of being traded for pot to kids who like to crush it and snort it. (Give your kids drugs and they will learn how to deal.)