The SAT Reasoning Test (formerly known as the SAT I test) is a standardized test that measures the critical thinking skills you will need for academic success in college. It is a three-hour mainly multiple choice test divided into three sections - Critical Reading, Writing and Math. Almost all colleges in the US require your SAT score in order to evaluate your application.
The SAT Subject Tests (formerly known as the SAT II tests), are multiple choice tests that measure your knowledge in particular subjects. Some of the subjects tested are: Math, Biology, Physics, English Language Proficiency, History, etc. Some colleges require or recommend that you to take the SAT II subject tests if you are applying for specific programs related to the subject.
Collegeboard is the official administers of the SAT tests. They set the test, conduct the tests and send score reports to each college
Visit their web site www.collegeboard.com for detailed information on each of the tests, study tips and other important information regarding the tests.