The SAT is a required test that has 5 parts and is sent to colleges when you are about to go to them. You take a PSAT = Practice SAT in sophomore year (optional) and a PSAT (mandatory) in junior year along with the actual SAT. The big difference between these two tests is that the ACT is more writing heavy while the SAT is not. These tests could be interchanged or taken together. It all depends on what the college you are planning on going to wants.