I'm 17, a junior in high school (born in December 88, will graduate in May 07, so I'll be 18 when I walk across that stage), and just took ACT yesterday. Here's the lowdown:
ENGLISH is a section wherein you read essays and sometimes stuff is underlined. Next to it are possible corrections if the grammar is wrong, or sometimes they throw in a "which of these options does NOT sound right?" to trip you up. Sometimes they also ask if a sentence should be deleted, moved around, or replaced. At the end of each essay they say "what if the writer wanted to write about (something), would this essay accomplish this?" and you say yes or no. That's it, repeated for about 4-5 essays on different topics (which sometimes can admittedly be interesting).
READING is a section where you read stuff and then answer questions about the content. Standardized tests start doing this sort of work in about the second/third grade, so everyone has been doing this for a LONG time by the time they meet ACT/SAT. Piece of cake, at least for me because I love reading.
MATH is a potent mixture of algebra, geometry, and small amounts of trigonometry. The trig is thrown in because they don't want everyone to get a perfect score. That's how all tests work--easy, medium, hard questions and they don't expect anyone to get a perfect.
SCIENCE is reading charts related to science stuff (chemical reactions, ecology, I got a really complicated part about planet rotations) and extrapolating certain data. Sometimes you have to speculate a little bit too.
Then there's the writing section, which just asks you to write an essay related to a certain prompt. Easy peasy.
The biggest difference, though, between ACT and SAT is thus:
The ACT does not penalize you for guessing.
"Well then, 10in2010," you might say. "Please elaborate!" And I will.
On all standardized tests, if you don't know the answer, you can guess and have a 1 in 4 chance of getting it right (or a 1 in 3 or 1 in 2 if you can eliminate the one ridiculous answer they generally throw in). However, on the SAT, if you guess wrong, points get deducted. The ACT works differently--only what you get RIGHT is scored. There's also a completely different scoring scale for the both of them: the SAT prefers 4-digit numbers and a perfect score is a 2400, the ACT prefers 2-digit numbers and a perfect score is 36.
My high school counselor recommends taking both because of that one big difference. The Board of Regents, who decides which minimum scores should be required for admission into any college here in Georgia, has minimum scores for both tests. The college I want to go to, Georgia State University, will take either test score as long as they include the writing part. I'm taking both tests, so I'll send in scores from both of them to the friendly folks of GSU admissions.
When registering for the tests themselves, you're prompted to give the names of a few colleges you want to receive your scores, so try to have a few potentials in mind before you start registering for tests. For example, I filled in Georgia State and University of Georgia, though now I don't want to go to UGA because they have a SERIOUS alcohol and drug problem on campus and there's nothing to do in Athens, the city UGA is in.
I'm taking the SAT in May.
I'm sorry if this reply is too long/irrelevant, but there's a lot of explaining to be done when it comes to standardized tests.