What is the difference between the ACT and 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 3 components:  Critical Reading, Mathematics, and a required Writing test.
The SAT penalizes you for wrong answers, so guessing is discouraged.  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.