In order to go to almost any 4-year college or university, a student must take one of the following entrance tests, SAT or ACT. Some of the more competitive schools also require that you take one or more SAT Subject Tests (SAT II) in addition to the SAT.

SAT is a general test.
SAT Subject Tests (SAT II) assess your abilities in specific subjects. Some schools identify which subjects in which you need to test; others give you the choice.
ACT is a test that assesses students' ability to complete college-level work with multiple choice tests in English, Mathematics, Reading, and Science... and an optional Writing test.
If you took an AP class this year, it might be wise to go ahead & take the SAT Subject Test in that subject this spring before you forget some of the material.