Web Crawlers

A web crawler, sometimes called a spider or spiderbot and often shortened to crawler, is an Internet bot that systematically browses the World Wide Web and that is typically operated by search engines for the purpose of Web indexing (web spidering).
Web search engines and some other websites use web crawling or spidering software to update their web content or indices of other sites’ web content. Web crawlers copy pages for processing by a search engine, which indexes the downloaded pages so that users can search more efficiently.
Crawlers consume resources on visited systems and often visit sites unprompted. Issues of schedule, load, and “politeness” come into play when large collections of pages are accessed. Mechanisms exist for public sites not wishing to be crawled to make this known to the crawling agent. For example, including a robots.txt file can request bots to index only parts of a website, or nothing at all.
The number of web pages is extremely large, and search engines do not index all web content. Studies of late-1990s search engines found that individual engines indexed only a fraction of the then-indexable web. Modern search engines use crawling, indexing, and ranking systems to return relevant results quickly, although not all pages are crawled, indexed, or served.
Crawlers can validate hyperlinks and HTML code. They can also be used for web scraping and data-driven programming.