The SATs are reasoning exams that test basic English and mathematical skills, so that colleges can measure your abilities compared to other students. They're mostly multiple choice with a short essay required for the English test. The tests are typically first taken near the end of your junior year in high school, and can be repeated as many times as you want in order to obtain the highest scores possible.
There are also SAT II subject tests, in which up to three different subjects (Literature, Math I and II, Physics, Biology E/M, Chemistry, world history, US history, and a multitude of foreign languages) are selected to be tested individually on. The SATs focus more on reading and logic skills, while the SAT IIs focus more on specific knowledge in a given subject area. However, only a handful of universities look for SAT II scores, while only about ten in the whole country require them.