The computer uses relevance ranking to place the queries which best match your search words at the top of the results list sent to you. The lists are usually created by human beings.
Some subject directories have annotations, and some directories rate sites.
Internet directories are usually arranged hierarchically by subject. Many directories have a query interface which allows you to search for terms in that subject hierarchy. Since no Internet search engines covers all of the millions of Internet web pages, meta search engines are a quick way to complete a broad search of the Internet.