Web spam refers to a host of techniques to subvert the ranking algorithms of web search engines and cause them to rank search results higher than they would otherwise. Examples of such techniques include content spam (populating web pages with popular and often highly monetizable search terms), link spam (creating links to a page in order to increase its link-based score), and cloaking (serving different versions of a page to search engine crawlers than to human users). Web spam is annoying to search engine users and disruptive to search engines; therefore, most commercial search engines try to combat web spam. Combating web spam consists of identifying spam content with high probability and – depending on policy – downgrading it during ranking, eliminating it from the index, no longer crawling it, and tainting affiliated content.
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