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 :-

https://www.google.com/amp/s/studyabroad.careers360.com/articles/what-is-sat/amp

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 :-

https://www.google.com/amp/s/studyabroad.careers360.com/articles/act-test/amp

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.