RoHS is a "blacklist" that explicitly restricts 10 substances in electrical and electronic products (lead, mercury, cadmium, hexavalent chromium, PBB, PBDE, and 4 phthalates), with fixed limits; REACH is a "dynamic list," with over 240 SVHC candidate substances and updates every six months, covering almost all products. RoHS only regulates electrical and electronic products, while REACH regulates all articles. Exports to the EU generally need to satisfy both.
Order of assessment: first check whether the product is electrical or electronic (if yes, RoHS is mandatory), then check whether it contains substances of very high concern (REACH applies generally). The two have overlapping test items, but their legal basis, limits, and notification obligations are completely different.