Google describes PageRank(PR) as the following:
"PageRank reflects our view of the importance of web pages by considering more than 500 million variables and 2 billion terms. Pages that we believe 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. We have always taken a pragmatic approach to help improve search quality and create useful products, and our technology uses the collective intelligence of the web to determine a page's importance."
It is saying that a PageRank results from a "vote" among all the other sites or pages. A site or a page that is linked to by many sites with high PageRank receives a high rank, and it wiil more likely to be listed at the top of the search engines. The quickest way to get high PR is to have some websites with high PR value linking to your site.