If rankings depended only, or even heavily, on on‐page factors, it would be quite easy to manipulate search engines. Since search engines don’t have a human understanding of meaning, it would be easy to create spam pages where key‐words appear in the right places (title, headings, bold text etc). Just a few years ago, it was not uncommon for a top result in Google to be an irrelevant page, where keywords were either hidden or used without relevant meaning.
In order to lower the ranking of such low‐quality results, off‐page factors have gained a great deal of importance in search rankings. In short, search engines now put a lot of weight on what other pages or websites think of a page.
Since spiders (and humans) navigate the web through links, search engines use linking as a way to determine the reliability of the linked page. By linking to an‐other page, a web page leads humans and spiders to it, so a backlink (inbound link to a page) may be considered a vote of confidence for the page, and the words which appear in the link text are used in determining what the linked page is about.
A page’s reliability or trust depends on a number of factors:
The authority of the domain: how reliable and trusted is the main domain? A page on Wikipedia, for example, is much more reliable than a page on a new or low‐traffic website. Domain authority is determined by a number of fac‐tors, including age, traffic and link popularity.
Number of backlinks: how many pages are linking to this page, and from how many different sources (domains)?
The authority of the linking page: how important is the page from which it is linked? A link from a high authority website (such as .edu or .gov sites) is more valuable than a link from an unreliable source such as a blog.
Google has its own method, known as PageRank™, of evaluating the backlinks to a page. Each indexed page is assigned a weighted number between 1 and 10 which signifies its link popularity. You can check the PageRank of a page here or by installing the Google toolbar. Note that PageRank is updated every few months and can sometimes vary unpredictably. So should not be relied upon.