SAT is a standardized test which is conducted in 65 countries across the world. Generally students who wish to study abroad take up this test but now things have changed since SAT is being accepted by many Indian Universities as well.
SAT mainly consists of questions from english, mathematics and grammar. It can be taken with or without an essay depending on the entry requirements of universities .
It is generally taken by students of age between 17-18 years for admission to undergraduate programs.
Hope this helps!