The SAT and the ACT are basically standardized tests that were (somebody please correct me if I'm wrong) put put together by a team of colleges. It is, basically, a summary of the knowledge you obtain through high school. It doesn't included upper level math, such as Calculus (I'm in it currently, but do not test well), but goes up to Algebra 2 and Trig. You are asked questions on science, math, reading, and... something else....

I can't remember right now.
The ACT is graded on a 36 point scale. A 36 is perfect, and football players get 1's.

Just kidding, I don't mean that. The national average on the ACT is, like, 19-21.
The SAT is similar, but graded on a 1600 point scale. I haven't taken it, so I can't tell what the average scores are.
As far as differences go, the ACT is basically the test used if you go to a midwest school. The SAT is used if your headed to foreign schools or schools on the coasts. Since I live in Missouri, I've only needed to take the ACT. I could use a higher score (as you may have read earlier, I got a 27) for scholarship purposes, but I know God will provide. Or at least He'll help me take out some loans.
Answer your question?
- Greg