Deep Web

What is the Deep Web
The term invisible web or deep web refers to the vast repository of information that search engines and directories don’t have direct access to, like databases at university libraries, sites that require passwords to view, or sites that for some reason don’t want search engines to crawl them. Unlike pages on the visible Web (that is, the Web that you can access from search engines and directories), information in databases is generally inaccessible to the software spiders and crawlers that create search engine indexes.

Learn more from an article published on lifehacker

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