Actually, its a damn good question considering many people don't differentiate between meth and speed. "Meth" is the common name for the chemical Methamphetamine, also called Desoxyn(brand name i think), whereas "speed" usually refers to Amphetamine(dexedrine a brand name, and adderall is a combination of four salts of amphetamine). These are two different drugs! Unfortunately, meth can be referred to as "speed" as well. The difference chemically is that methamphetamine has an extra methy group attatched to the amphetamine molecule. I would suggest looking in erowid for more information on these two molecules, there is quite a wealth there. The timelines are pretty useful for understanding the history of these drugs(and their military and pharmaceutical uses-apparently soldiers on all sides used both in WWII to increase performance).
Be aware, know your drugs.