SAT I is a paper-based general test, assessing math, reading, writing and language and essay skills. Most US Universities require SAT Scores for admissions. SAT I is a 3 hours and 50 minute multiple choice test with the essay and 3 hours test without the essay. It has a validity of 2 years. SAT is scored out of 1600: a separate score of 800 each for Mathematics, Reading and Writing while the essay is scored separately on a scale of 2 to 8.
Many universities also require students to take SAT-II Tests. It has a validity of two years. Each SAT II Subject Test is of one-hour duration, scored out of 800. SAT II is only required by top colleges. SAT II is conducted for the following subjects: Literature, American History and Social Studies, World History, Math IC, Math IIC, Biology, Chemistry, Physics and different languages like Chinese, German, French etc. Check with your college whether they want it.