The SAT Subject Tests are a collection of 20 different tests within 5 specific categories: English, History, Languages, Mathematics, and Science. Each subject test lasts 1 hour and consists entirely of multiple-choice questions.
Unlike the SAT Subject Tests, the SAT is largely a test of verbal and math skills. Although you need to know vocabulary and a few formulas, it’s primarily designed to measure how well you read and think, rather than what you know.
The SAT Subject Tests, on the contrary, are designed to measure what you know about specific subjects. Sure, critical reading and thinking skills play a part, but the main purpose of the Subject Tests is to determine exactly what you know about math, history, chemistry, and so on.