Google PageRank or PR for short is numeric value assigned by Google to pages that represents how important a page is. The more important and known your pages are, the better PR they will get. PR is a scale from 0 to 10 where 0 is an unknown page for Google and 10 is the highest PR that can be assigned.
PageRank provides the basis for all of web search tools. It is always updated by Google in order to improve it and optimize it.
