Link Popularity, On-Page Characteristics, and Content Analysis.

Several search engines use link popularity to some extent. Google's PageRank is the original form and remains its purest example.  On-page characteristics include, but are not limited to, such things as font size, headings, word frequency, and domain name.  Content analysis usually involves the search results being grouped into categories that allow the user to drill down for more specific results. Each method is not without merit. Search engines use some combination of the first two; some use on-page characteristics alone, and some even combine all three methods. 

An update to the PageRank algorithm called “the Hilltop algorithm” was implemented around the end of 2002 that changed a few of the defining factors.

Hilltop also requires that it can easily locate at least 2 “expert documents” voting for the same Web page. If it cannot find a minimum of 2 such "expert documents", the results returned will be absolute zero. This means that Hilltop actually refuses to pass on any arbitrary values that may be relevant to the rest of Google's ranking formula.

Google also uses the Hilltop algorithm to better define how a site is related to another, such as in the case of affiliate sites or similar properties. The Hilltop algorithm is the technology that allows the detection of sites that use heavy cross-linking or similar strategies. Additionally, research has indicated that an OOP (Over Optimization Penalty) could be applied to sites with too much emphasis of their main keywords.