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. Within each category, nonfiction books are shelved in numerical order by their Dewey Decimal call numbers, making it easy to find related books on the same subject near each other.