What is the difference between the ACT and SAT?

Great Resource for Comparison:
http://www.studypoint.com/ed/act-vs-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 SAT  takes off for wrong answers.  

The ACT is scored based on the number of correct answers with no penalty for guessing.

ACT questions tend to be more straightforward. ACT questions are often easier to understand on a first 
read. 
On the SAT, you may need to spend time figuring out what you're being asked before you can start solving the problem. For example, here are sample questions from the SAT essay and the  ACT writing test (their name for the 
essay):
SAT: What is your view of the claim that something unsuccessful can still have some value?
ACT: In your view, should high schools become more tolerant of cheating?

Colleges will accept either the SAT or ACT. So which should you take?
It is highly recommended that you take BOTH the SAT and ACT your Junior year.  

How do the scores compare?
ACT and the College Board have completed a concordance study that is designed to examine the relationship between two scores on the ACT and SAT. These concordance tables do not equate scores, but rather provide a tool for finding comparable scores.