What is the difference between the SAT and the ACT?

The following information was taken from the ACT website. (FAQ: ACT/SAT)

"The ACT is an achievement test, measuring what a student has learned in school. The SAT is more of an aptitude test, testing reasoning and verbal abilities.

The ACT has up to 5 components: English, Mathematics, Reading, Science, and an optional Writing Test. The SAT has only 3 components: Critical Reasoning, Mathematics, and a required Writing Test.

The College Board introduced a new version of the SAT in 2005, with a mandatory writing test. ACT continues to offer its well-established test, plus an optional writing test. You take the ACT Writing Test only if required by the college(s) you're applying to.

The SAT has a correction for guessing. That is, they take off for wrong answers. The ACT is scored based on the number of correct answers with no penalty for guessing.

The ACT has an Interest Inventory that allows students to evaluate their interests in various career options."
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In Gruber's Complete SAT Guide 2011, Dr. Gruber answers the question, SAT vs. ACT: How Should students Decide Which Test To take? His answer follows.

   "The correlation happens to be very high for both tests; if you score well on one, you will likely score about as well on the other. They cover a lot of the same material. Both exams test grammar, math and critical reading skills. However, the ACT includes a whole section on scientific data interpretation (the SAT has a few similar questions in it's Math sections); fortunately, you don't have to have a scientific background to excel on the ACT.
   The ACT is more memory-oriented, while the SAT is more strategically-oriented. If you memorize quickly and retain facts well under pressure, I recommend the ACT. If you are more prone to strategizing or you like puzzles, I would take the SAT. In any event, I would check with the schools that you are applying to and find out which test they prefer."
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*Also worth noting is that students get 25 minutes to write an essay for the SAT while they get 30 minutes for the ACT essay.