Page Rank (often denoted by PR) is Google's ranking software that calculates the relevance of a webpage with its content. Page Rank is a quantity (between 0 and 10) defined by Google that provides a rough estimate of the overall importance of a web page.
Page Rank is displayed as a green bar to the left of a webpage listing in the Google Directory, and also in the Google toolbar. The basic factor behind defining a page rank is the keywords although there are "over 200 SEO factors" that Google uses to rank pages in the Google search results (SERPs).
The Page Rank of a website measures not only the number of links that points to a website, but the quality of the websites that provides the links. For a website to have a high page rank, it must have a lot of quality back links. If a website contains a lot of useful information without containing quality back links, the website will definite rank low.
The technology behind page rank is still hidden to many, although many seo gurus still believe that one can manipulate the page rank of a webpage. Yes this can really be done. It will only take one hard work and time. In summary, the page rank of a webpage is a measure of the importance of a page.