What's the difference between ACT and SAT in America?

As far as I know both of the tests are for high school students who want to go to college.

SAT stands for Scholastic Assesment Test and

ACT stands for American College Test

I would be grateful if you could tell me the difference.

The SAT has 3 sections - Reading, Writing, and Mathematics, which are each scored out of an 800-point scale (granularized in 10-point units), with the mean normed to 500 points. The SAT is taken more frequently than the ACT by students on the East and West coasts.

The ACT has 4 sections - English, Reading, Mathematics, and Science Reasoning - as well as an optional Writing section. The ACT offers individual scoring on each section as well as an overall average called the composite score. The 4 mandatory sections are scored from 1-36, with the composite score originally normed to a mean of 18 points (with integer scores only). The Writing section is scored from 2-12; I don't know what the mean is normed to (if it's been normed yet at all). The ACT is taken more frequently than the SAT in the middle of the country.
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Thank you very much for your help.

Do all American schools accept both scores?

I have the impression that, SAT is more common, becase its form is similar to GRE.

I'm sure there are still some colleges that only accept one or the other, but most accept both.
You would definitely want to check the admission requirements for the college you were interested and find out which one they favored, however. I took only the SATs. I didn't even know what the ACTs were until much later.
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Thanks Grammar Geek for your explanation.

Have you taken GRE?

I took GRE last year. It was very difficult for me. There were ful of new words despite the fact I have a rich vocabulary.

I wonder if native speakers also find it difficult.

I did not need the GRE for my MBA program, so no, I have never taken it. For a while I thought about going back to get a different master's degree and looked briefly at some sample tests for the GRE, but never pursued it. I remember thinking that it was not a simple test. (I thought the SATs were unbelievably easy.)