Since high school curriculum and grades are not standardized - at least until common core- , the SAT and ACT give colleges a uniform national benchmark to compare student capabilities. The SAT and ACT are administered by two separate entities based in NJ (College Board, SAT) and and Iowa (ACT). The College Board administers the PSAT, SAT, and SAT subject tests and AP exams. CB management was also involved in common core. ACT administers Explore, Plan, ACT and several others.
Years ago the SAT was popular on the coasts and ACT in the interior. The ACT has surpassed the SAT in popularity. Colleges do not care which one you take.
The ACT has 4 sections- english, math , and reading but also includes a ‘Science’ section, which is actually a more of a data interpretation section. There is also a composite score which is the average of all 4 sections rounded up. You don’t need to know any science to complete it.
Most students do better on one than the other. ACT questions are more straightforward. (This will likely change with the new SAT). The old SAT (SAT I) has 10 sections and takes over 4 hours to complete. The ACT has 4 sections and takes closer to 3.5. The ACT also has a writing option which some colleges want. All are questions are multiple choice except writing. SAT subject tests (also called SAT II) are short (45 min) and include 20 separate tests in Social Studies, Math, Science, Language reading and Language listening tests categories. You can take up to 3 in a given day.
Each tests costs around $50, less for the SAT subject tests, more for late registration. Low-income fee waivers are available. You also get (usually) 4 free reports sent to colleges, and a free one to you and your high school. There are some limitations, mostly that you have to select the schools before/during the tests for free reports. Extra reports cost $12 from ACT.
Each college has different requirements. Some want ACT w writing or SAT, some want subject tests, some don’t care. Some accept AP credit, others do not, or require a certain score. Programs like engineering typically require a high math score on ACT/SAT and subject Tests.
My advice to a Sophomore would be to prepare for the PSAT first, given in Sept/October to juniors, as best you can (with no prep material for the new test) . If you do reasonable well for your state you will also need to take the SAT and fill out an NMS app. Otherwise I would concentrate on the ACT for now.