TrustRank is a link analysis technique that several search engines, including Google have been using for semi-automatically separating useful web pages from spam. This makes it a lot harder for black hat search engine techniques to succeed because the human element is involved when it comes to assessing web pages. TrustRank is particularly successful when it comes to weeding out web pages that are created clearly for commercial reasons with the absolute intention of misleading search engines and uses a combination of human assessment and automated techniques to selectively identify bad pages.
Many Web spam pages are created only with the intention of misleading search engines. 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 evaluate manually a large number of pages. TrustRank is a hit and miss type of system that uses random sampling to try and keep crap off of the Internet. If you are practicing black hat SEO you need to watch out for TrustRank!
One popular method for improving rankings is to increase artificially the perceived importance of a document through complex linking schemes. Google's PageRank and similar methods for determining the relative importance of Web documents have been exhausted and corrupted by such manipulation. This makes it difficult for good practitioners of SEO as it forces the big search engines to constantly change their algorithms. The TrustRank method calls for selecting a small set of pages to be evaluated by one human expert. Once the credible pages are manually identified by a human, a spider is sent out to find credible pages that are similar. This helps prevent spam associated with the exploitation of certain keywords and niches and forces web masters to come by their page rankings in Google and Yahoo in an honest fashion.