What is the SAT?   The SAT is a three-hour, primarily multiple-choice test that measures verbal and mathematical reasoning abilities that develop over time.  Many colleges require SAT scores for admission.

What are the SAT II subject tests?  Subject tests are one-hour, primarily multiple-choice tests that measure your knowledge of particular subjects and your ability to apply that knowledge.  Check the requirements of the colleges you're considering.  Many require or recommend one or more Subject Tests for admission or placement.