Definition: A web site that offers its visitors the ability to search the content of numerous web pages on the Internet. Search engines periodically explore all the pages of a website and add the text on those pages into a large database that users can then search. With a search engine, publishing web pages that incorporate relevant key phrases, prominently positioned in particular ways, is critical. Contrast this with directories, which don’t siphon content out of the HTML of a site’s constituent pages, but instead are comprised solely of site names and descriptions written or edited by human reviewers
FirstClickSEO Commentary: The most popular is Google, followed by BING, and Yahoo is bringing up the rear.