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Web search engines use software robots called spiders to search the Web and build lists of keywords found on Web sites. This is called Web crawling. These keywords are kept in an index along with where these words are found. When Google's spiders look at a Web page, it indexes every significant word on the page. The main difference between Google and other search engines is how the pages are ranked.
Google uses PageRank™ technology to rank Web pages. How a page is ranked is determined by the frequency and location of a keyword within the page, how long the Web page has existed, and how many Web pages link to that particular page.
For more information on how this works, see How Google Works.