PageRank is not the only algorithm Google uses to determine relevancy of pages. In this article we discuss two more publicly known algorithms, Hilltop and Trust Rank. Keep reading for some enlightenment on how the world's most popular search engine ranks web sites.
Both algorithms are quite old in the search engine arena (2005), but are still in use, as is the initial PageRank algorithm (though much tweaked over time). You need to be aware of both in order to understand search engines better.
In this article we discuss Hilltop and TrustRank, the reasons behind those algorithms, and how to optimize better with each algorithm in mind. We also link to the official white papers that cover each algorithm and useful resources so you can learn and become more effective in SEO game.
…we propose techniques to semi-automatically separate reputable, good pages from spam. We first select a small set of seed pages to be evaluated by an expert. Once we manually identify the reputable seed pages, we use the link structure of the web to discover other pages that are likely to be good….
To summarize how Trust Rank works, a page is first identified by a human as being of very high quality, or a "seed page." Pages that the "seed page" links to are classified as high quality pages as well. Pages that are linked from seed pages get high Trust Rank, and the further a page is from a seed page in the web's link structure, the less Trust Rank is passed on to it.
Once seed pages are identified, it is easier to separate spam from quality pages. The logic behind Trust Rank is the following:
Quality pages do not link to spam (usually).
The trust score is reduced as it passes from page to page.
The most “powerful” trust score is from a seed page itself.
Google Optimization 
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