Page Rank (PR) is a numerical value characterizing authority of a Web page in the Google. It's being computed, using data about external factors of a page – the more authoritative sites reference per a page, the more authoritative itself becomes.
As a result, the PageRank is the method of a page weight calculation by computation of page references importance. The PageRank is one of the auxiliary factors participating during a site ranking in search results.
One can specify the PageRank pages having downloaded free Google ToolBar; in search it's shown in form of a green stripe with values from 0 to 10.