Search engines send out automated programs - spiders , which use the web’s hyperlink structure to “crawl” it's pages.
After spiders crawl a page, its content needs to be put into a format that makes it easy to retrieve when a user queries the search engine. Thus, pages are stored in a giant, tightly managed database that makes up the search engine’s index. These indexes contain billions of documents, which are delivered to users in mere fractions of a second.
When a user queries a search engine, the engine examines it's index to find documents that match.
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