Like those above me have said, the SAT and ACT are accepted by different schools nationwide, but their scoring systems are different as well, like the SAT will take away points for a wrong answer, but no points are deducted for no answer or leaving the answer blank where the ACT has a different system associated with it.
The SAT also has some “Subject Area Tests” associated with it that some schools want you to take, whereas the ACT is pretty much just the ACT.