Okay..I am in the midst of the college board process and I know all about this. The ACTs and SATs are two completely different tests. The SATs have a Critical Reading, a Math, and a Writing (half grammar half essay) section. Each is worth 800 points; a perfect combined score is 2400. Because the Writing section is new, most colleges view it with the same weight as an SAT Subject Tests. (These come in many subject areas like Spanish, Chemistry, etc. Most schools like to see a few to supplement the SATs.) The really importnt numbers are your Math and Critical Reding scores. The ACTs are scored out of 36, so it's a little more holistic. This test includes science, and the atual essay is optional but recommended.
Most colleges do NOT require both! They all say that they require EITHER one, but the SATs are a little more common amongst appliants to the better schools. Both tests are available to everyone, but the ACTs are more of a Midwest thing..east and west coasters generally stick with SAT scores.
AP Tests are completely different. High Schools off Advanced Placement courses that are supposed to simulate the experience of a college course. Serious students these, and although you are not obligated to take them, if you want to get into a really good school you must prove that you have challenged yourself. These tests are given in May and are scored 1-5. A 3 is considered "good," but you really want fours and fives. Colleges sometimes offer credits for doing well on AP exams.
To the other poster..writing IS required on the SATs. It's a whole section now.
Heh I actually just took a subject test this morning. It was really easy..