Adderall is amphetamine, the base compound.

Meth is methamphetamine, amphetamine with a methyl group attached to it, so it's very closely related to Adderall. That small change makes it a bit more potent, longer lasting and euphoric though.

But meth can also get prescribed for ADHD and narcolepsia (under the brand name 'Desoxyn') in very rare cases where even Adderall doesn't seem to do the trick. So basically, both substances can be used therapeutically and they both can fuck up your life if you start abusing them, with meth admittedly being more potent and dangerous than regular amphetamine.