The ACT and the SAT are college admissions tests required by most colleges. Most college admissions committees will evaluate your application based on your best single-session score, so don't hesitate to send scores from multiple test days. For those of you who don't test well, you'll be glad to know that there are colleges that are test-optional (You can view a list on the FairTest website). However, you may still need to take an ACT or SAT, but the score is not a critical piece of the application.
Most colleges accept ACT or SAT scores, so you may wonder if you should take the ACT, SAT, or both exams.
The ACT includes multiple-choice subject tests covering English, Math, Reading, and Science, designed to evaluate test-takers' overall educational development and ability to complete college-level work. Students have 2 hours and 55 minutes of dedicated test time to complete the subject tests, not including breaks. The ACT also includes an optional 30-minute writing test designed to measure a student’s skill in planning and writing a short essay which gives a student a chance to highlight their writing skills! If you opt to take it, the additional scores will be reported, along with comments about your essay. These scores are reported separately.
The SAT is now much more like the ACT than it was in the past. The SAT has one Reading section and one Writing section and the Math section is divided into one calculator portion and a no-calculator portion. Unlike the ACT, there is no Science section. The test is 3 hours plus the optional essay.
Find out more about both tests here: http://blog.prepscholar.com/the-new-sat-vs-the-act-a-full-breakdown