Google pagerank explained:

PageRank is the measure of importance Google assigns to a Web page on a scale of 1 to 10.
You can check the PageRank value of every page you visit by downloading the Google Toolbar.

Google’s founders, Sergey Brin and Larry Page, formulated a Search Engine algorithm that shifted the ranking weight to off-page factors. They developed a formula called PageRank, in which the algorithm counts the number of sites that link to a page and then assigns the page an importance score on a scale of 1-10. The higher the number and importance of sites that link to your page, the higher the PageRank of your Web site.

This algorithm has not only helped provide authentic and quality information, but also has made it very difficult for site owners and webmasters to cheat their way to top rank.