Students who are applying to study in America would normally sit one of two admission tests - the SAT or the ACT. Taking at least one of these is required by most (but not all) universities in America.
The SAT reasoning test comprises two sections - Mathematics, and Critical Reading and Writing. There is also an optional essay. In addition to the SAT reasoning test, some universities (usually the more competitive ones) will require students to sit 2 or 3 ‘subject’ tests. It would usually be sensible for students to take these tests in subjects that they have studied at school and in which they are therefore most knowledgeable. SATs can be taken at various times throughout the year at any of a number of centres throughout the UK. Students often sit their admissions tests in year 12 so that if they are not happy with their grade they will have the opportunity to sit the test again later.
The ACT has four compulsory sections - English, Mathematics, Reading and Science Reasoning. There is an additional Writing section which might be required by the more competitive American universities. As the ACT contains a Science section and a more rigorous Maths paper than the SAT, students who are good at these subjects may well benefit from sitting the ACT.