PageRank is an algorithm used by Google to measure importance of a web page. Since there are billions of pages on the web, google had to find a way to make the difference between them, to place them in order, to rate each page.

PageRank is actually a number, between 0 and 10. So the best page has page rank of 10. There are some pages that have page ranking 10 (Adobe, Macromedia, Apple, etc).

Google Page Rank is determined by number of quality backlinks. If a webpage A has a link to another page B, is like webpage A voted for page B. The more backlinks for a web page, the higher is the pagerank. And another factor for page rating, is the quality of the page A. The higher pagerank for page A, the higher importance in pagerank’s calculation for site B. Simple, clear and logic.

Read more about Google Page Rank on Google official website or on Wiki

Want to improve your ranking? Read this tutorial

http://marketing.askbee.net/google-page-rank-checker-tool/