SAT refers to as Scholastic Aptitude test . It is an international level entrance examination whose score is accepted by many reputed Institute like MIT , Harvard , Yale University etc .
You can check complete details in the link given below :-
ACT refers to as American College Testing it is an entrance examination which helps you take admission in Colleges in USA , you can check details through this link :-
The difference between the two SAT score is accepted by various Institutes all over the world but ACT score is only accepted by USA Institutes.