Colleges and universities use equally the SAT and the ACT to evaluate prospective students. It is recommended that students take the SAT and/or ACT once during the spring of their junior year and once during the fall of their senior year.  There are also schools that don't require either exam as part of their admissions requirements.  Here is a  List of Test-Optional Schools.

The ACT is a national college admissions test that consists of subject area tests in english, mathematics, reading and science.

The SAT (Scholastic Aptitude Test) is a multiple-choice and essay test that tests verbal and mathematical aptitude as well as writing skills. ar.

The PSAT (Preliminary SAT) is a standardized test that provides practice for the SAT. For juniors, the PSAT is also the qualifying test for National Merit Scholarship programs. PSAT scores are not sent to colleges.

The SAT Subject Tests measure knowledge actually gained in the classroom (i.e. achievement rather than aptitude). SAT Subject Tests consist of one hour multiple–choice tests in specific subjects: American History and Social Studies, World History, Biology, Chemistry, Physics, English Writing, English Literature, Chinese, French, German, Hebrew, Latin, Spanish, Mathematics Level I (basic math with some trigonometry) and Mathematics Level IIC (with some advanced algebra and calculus). Not all colleges require the Subject Tests for admission or placement purposes. No college requires more than three SAT Subject Tests, though a student may choose to take any number. It is your responsibility to find out how many and which SAT Subject Tests are required by the college to which you are applying. These tests should be taken at the end of a one–year course, such as biology, and as late as possible in continuing courses, such as foreign languages. Students who are Early Decision/Action candidates may be required to complete three SAT Subject Tests by the June testing date of their junior year.