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What is Web crawler?

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A web crawler (also known as a web spider or web robot) is a program or automated script which browses the World Wide Web in a methodical, automated manner.
This process is called Web crawling or spidering.
  Many legitimate sites, in particular search engines, use spidering as a means of providing up-to-date data.
  Web crawlers are mainly used to create a copy of all the visited pages for later processing by a search engine, that will index the downloaded pages to provide fast searches.
  Crawlers can also be used for automating maintenance tasks on a Web site, such as checking links or validating HTML code.
  Also, crawlers can be used to gather specific types of information from Web pages, such as harvesting e-mail addresses (usually for spam).
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