I is a three-hour test, primarily multiple choice, that measures verbal and mathematical reasoning abilities. The SAT II are one-hour, mostly multiple choice tests in specific subjects. These tests measure knowledge of particular subjects and the ability to apply this knowledge.
Many colleges require or recommend one or more subject tests for admission.
So check if you have to take any of these tests. While most colleges expect applicants to take the SAT, there are a few colleges where a SAT score is not mandatory.