In fact, both are necessary, because directories created by humans are incomplete, and search engines that probe the Web's millions of pieces of constantly changing information lack the human intervention that can help users find the most relevant results. They search for files related to a specified query by key word or narrowed search string.
Directories provide reasonable quality control but a more limited selection of sites.
Yahoo is the most famous subject directory. Natural Language Interfaces are a relatively new development. The user simply asks a question. Second Generation Search Engines use evolving technology to organizes search results by concept, site, domain, popularity and linking rather than by the more relevancy. To learn more, visit the Finer Points .