PSAT is a preliminary test and is meant to give you an idea of how well you’ll score on the SAT or ACT tests. The PSAT score does not count for college admission.
The ACT math test has algebra II, trigonometry and some geometry questions that are absent from the PSAT math test. The overall difficulty of ACT math is higher than that of PSAT. There are no questions with logarithms, quadratic equations, trigonometric functions in PSAT.
If you are a junior, a high PSAT score can help you to be considered for the National Merit Scholarship Program.
The PSAT has multiple choice and grid-in questions while the ACT has only multiple choice questions.