Page Rank is a unique score out of ten that Google gives to every web site on the internet. A high rank of 10 means a very important website - in fact only 9 websites in the world as of this moment have a rank of 10.
On the other hand, out of 240 million websites - the majority of them are page rank 0 and 1 is the mean score.
The higher your page rank, the better your site scores in search results for relevant keywords, so here is my very basic pagerank calculator. Each link to your site is worth a vote and some votes are more important than others. A link from a rank ten site automatically gives your site an 8. To get a ten from page rank 1 votes however requires almost an infinitesimal number of votes.
