PageRank™ (PR)
PageRank is Google's method for measuring the relevance or importance of a web page. It is based on a scale from 0 to 10, where 10 is the highest.

The main factor behind PageRank is link popularity. When one page links to another page, Google sees this as a "vote" for the other page. The more votes that are cast for the other page, the more important the page must be.

The importance of the page casting the vote determines how important the vote itself is. So Google sees a web page with a higher PageRank as a more authoritative source for casting a vote.