this is from google -
http://www.google.com/corporate/tech.html
PageRank Technology: PageRank reflects Google's view of the importance of web pages by considering more than 500 million variables and 2 billion terms. Pages that Google believes are important pages receive a higher PageRank and are more likely to appear at the top of the search results.
PageRank also considers the importance of each page that casts a vote, as votes from some pages are considered to have greater value, thus giving the linked page greater value. Important pages receive a higher PageRank and appear at the top of the search results. Google's technology uses the collective intelligence of the web to determine a page's importance. There is no human involvement or manipulation of results, which is why users have come to trust Google as a source of objective information untainted by paid placement.