Unlike a search engine, a directory or rating service is based on people who evaluate web content. Sites are assigned to an appropriate category and often provide more relevant results. Categories might include Entertainment, News, Arts, etc. When a user enters a few keywords into the search engine or directory service, the program looks through its catalog and finds web sites that match the keywords. Most search engines then display the ten most relevant matches and give you the option to view the rest.
Every search engine determines what is relevant using different criteria.
Some use the frequency of visitations to the site, while others check for the number of times your keywords are mentioned in certain parts of a web site document.