A web crawler is a program that browses the World Wide Web in a methodical, automated manner. This process is called web crawling or spidering. Many search engines use web crawlers to update their indexed lists of websites. When a search engine visits a website, it takes note of the words used on the site and any links leading to other websites. The search engine then uses this information to create an index, which is used when someone types in keywords associated with those terms.
Web crawlers can be written in any programming language, but most are written in Perl or Java. They typically run on central servers and visit new sites as they are added to the Internet and re-visit existing sites at regular intervals to check for changes. When they find new or changed content on a site, they update their records accordingly so that users will be able retrieve this updated information when they perform searches using the relevant keywords.
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