The trend to define which information is more trustworthy has shifted from meta tags and keywords toward links and contents. Take Google’s PageRank for example. “In essence, Google interprets a link from page A to page B as a vote.” Of course, PageRank will also value a link to determine its importance. 5 factors are listed as the following:
1. The anchor text of link
2. Global popularity of the site
3. Popularity of site in relevant communities
4. Text directly surrounding the link
5. Subject matter of the linking page
Source: SEOmoz.org