Some engines use the popularity of sites as part of a ranking mechanism as they can determine which pages have plenty of links pointing at them. Search engines vs directories A directory looks just like a search engine, but it's compiled by human researchers, not robots. And it's the human factor that sets a good directory like Yahoo apart from search engines. Directories are likely to return much fewer results to any search than a search engine, but the content of the results is usually far better. Directories are also the better choice if you are researching a fairly broad subject.
Some directories also supply site reviews that help in the quest for quality content.
Different search engines are dedicated to difference aspects of the web. The best known are dedicated to web content, but if you are looking for mailing lists, e-mail addresses or Usenet groups you'll be better off with a specialist engine. For instance, DejaNews lets you search for a particular newsgroup on Usenet. The largest search engines and directories are much more than just search tools these days. They may offer free e-mail, news, software, and pages that can by customised to suit your personal requirements.