The document discusses the Dewey Decimal Classification system, which was created by Melvil Dewey in 1876 to organize books in libraries. It divides all knowledge into 10 main categories represented by three-digit numbers. Each book has a call number based on its subject matter that can be found on the spine, allowing books to be easily shelved in order. The 10 main categories are General Knowledge, Philosophy, Religion, Social Sciences, Languages, Natural Sciences, Technology, Arts, Literature, and Geography/History.