What’s the main difference between this two?
if you want to apply the graduate degree of US universities or colleges, you have to participate GRE(Graduate Record Examination),which is held by ETS.GRE is not a simple examination, which could overwhelm you. So before you decide to continue your study in Graduate School of United States, you need enough preparation.
Some colleges require international undergrad applicants to take GRE subtitude for SAT if SAT test centers are not available where they from. That really seems unfair because if you can do well on GRE, what are you going to college for?