The document discusses the Dewey Decimal System, which was invented by Melvil Dewey to categorize books into 10 main subject groups represented by 3-digit numbers. It explains the general categories including 000s for general works, 100s for philosophy, 200s for religion, and so on up to 900s for history and geography. Nonfiction books are organized on shelves first by their Dewey Decimal number, which helps readers find books on the same subject near each other.