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.