Google PageRank is based primarely on the “votes” submitted by other web sites under the form of the links. It uses a complex algorithm to assign values, in order to measure the relative importance for every hyperlinked element.
An interesting article was posted on Google Blog by Amit Singhal, a Google Fellow in charge of the ranking team at Google.
He reveal three most important principles in Google ranking: 1) Best locally relevant results served globally, 2) Keep it simple, and 3) No manual intervention.
It means that the results for local query will be used to improve the global search, the search system has to be kept simple, and that Google uses humans to improve and complete the algorithms only, no manual intervention in the final results.
Google itself says that “PageRank relies on the uniquely democratic nature of the web by using its vast link structure as an indicator of an individual page’s value. In essence, Google interprets a link from page A to page B as a vote, by page A, for page B. But, Google looks at more than the sheer volume of votes, or links a page receives; it also analyzes the page that casts the vote. Votes cast by pages that are themselves “important” weigh more heavily and help to make other pages “important”.“
So, the final rank is the result of interaction (links) from all the other web sites, based on the importance assigned to each link.
The PageRank is like a mark weighting between 0 and 10, in a logarithmic scale.