TrustRank is a new technique proposed by researchers from Stanford University and Yahoo to semi-automatically separate reputable, good pages from spam.
Many Web spam pages are only created with the intention of misleading search engines. See also Spamdexing. These pages, chiefly created for commercial reasons, use various techniques to achieve higher-than-deserved rankings on the search engines' result pages. While human experts can easily identify spam, it is too expensive to manually evaluate a large number of pages. Therefore, Google first selects a small set of seed pages to be evaluated by an expert. Once the reputable seed pages are manually identified, Google uses the link structure of the web to discover other pages that are likely to be relevant and good. Google claims that they can now effectively filter out spam from a significant fraction of the web, based on a good seed set of fewer than 200 sites
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http://en.wikipedia.org/wiki/TrustRank