Assuming the SAT Reasoning Test (as opposed to Subject Tests)
SAT has three categories: reading, math, writing. The test is divided into 10 sections (which really bothered me on the actual test). Here is the overall structure of the test:
Section 1: Essay (writing)
25-minute essay on a given topic, usually moral or philosophical in order to appeal to the entire student population (e.g. "Do leaders of societies tend to overuse their power").
Sections 2-10: Reading, math, writing (in random order, but Section 10 is always a short writing section. Each section usually lasts 20-25 min)
Reading:
Vocabulary, comparing/contrasting two short stories, "long" reading sections
Writing:
Essay, grammar, proofreading
Math:
algebra, geometry, probability, some introductory statistics