There is a difference. Adderall is a combo of amphetamine and dextroamphetamine, while "meth" is typically mostly methamphetamine (with other "enhancements" or impurities). They are chemically different.

While Adderall isn't the same as meth, there is a ADHD medication that is pure methamphetamine: Desoxyn, and it's significantly more difficult to get a prescription for, and comes with far more warnings about its use since it is pure meth. I've never tried it, nor have I ever had a doctor recommend it.

The doctor who I was seeing when I was first diagnosed with ADHD explained the differences between Adderall and meth by saying "Adderall is like a bright line painted on the ground helping remind you of where to go. Meth is like riding a bullet train. It'll lock you in tight and take you for a ride, hurtling at full blast whether you want to be a part of it or not until you and everything you love shakes to pieces and you die. And it'll tell you it's fun while it's happening."

I had a friend who'd been addicted to meth and is in recovery say "Adderall is like taking a comfortable stroll around the block, while Meth is like travelling to the moon and back. Yeah, they're both 'getting you somewhere', but they're not remotely the same activity."