First, for each of the tests, your raw score is calculated. Your raw score is simply the number of questions you answer correctly.
The raw score is then converted into a scale score. There is a scale score for each test: English, Mathematics, Reading, and Science Reasoning, ranging from 1 to 36. The system of converting a raw score into a scale score varies slightly from one test battery to another, since the test forms vary slightly in difficulty. If you get a slightly harder ACT, the score conversion table for your exam will be a little easier. You'll be able to get a couple more questions wrong while still earning the same scale score. The test-makers have an elaborate system based on statistical theories for figuring out exactly how much they need to compensate for slightly harder or easier test forms. The idea is to make any given score represent the same level of difficulty no matter when or where the test is given.
Next to each scaled score is a percentile ranking. Percentile ranking refers to the percentage of people who performed better or worse than you did on the test. For instance, a percentile ranking of 87 indicates that 86% of the people who took the test scored lower than you did, and 13% scored higher. (86 + 13 +1 (you) = 100%)
You'll also receive subscores for particular subcategories on three of the tests. In English, you'll receive subscores in usage/mechanics and rhetorical skills; in Mathematics, you'll receive subscores in pre-algebra/elementary algebra, algebra/coordinate geometry, and plane/geometry/trigonometry; and in Reading, you'll receive subscores in social studies/natural sciences and prose fiction/humanities. There's no subscore in the Science Reasoning test. The subscores are reported on a scale of 1 to 18. They are also reported as percentiles. The subcategories may be of some marginal use to colleges; they are much more useful to you if you decide to take the test again, as a way to pinpoint your strengths arid weaknesses.
Finally, at the bottom of the page, you will find a composite score -- one overall number that summarizes how you did on the ACT. This is an average of what you received on the four main sections, and is the most important score for most test-takers. The composite score is used by most colleges and universities in the admissions process.