From Danny Sullivan’s “What Is Google PageRank? A Guide For Searchers & Webmasters” (Search Engine Land: 26 April 2007):
Let’s start with what Google says. In a nutshell, it considers links to be like votes. In addition, it considers that some votes are more important than others. PageRank is Google’s system of counting link votes and determining which pages are most important based on them. These scores are then used along with many other things to determine if a page will rank well in a search.
PageRank is only a score that represents the importance of a page, as Google estimates it (By the way, that estimate of importance is considered to be Google’s opinion and protected in the US by the First Amendment. When Google was once sued over altering PageRank scores for some sites, a US court ruled: “PageRanks are opinions–opinions of the significance of particular Web sites as they correspond to a search query….the court concludes Google’s PageRanks are entitled to full constitutional protection.)