The Google search engine uses a link analysis algorithm called PageRank to assign a relative numerical importance to a set of hyperlinked documents, such as the World Wide Web.
For a given page, it's importance (the PageRank value) results from a ballot among all the other pages in the set. For a page to give a vote to another, it must link to it, and so the PageRank depends on the number of incoming links, anf the PageRank of those pages that provide the links. Pages that are linked to by many high ranking pages will themselves obtain a high rank. If a page has no incoming links, there is no support for that page.
The PageRank is a numeric weighting of 0 to 10, and denotes how important a site is in Google's eyes. Like the Richter Scale, the PageRank is a value on a logerithmic scale that is derived from a probability. In addition to the quantity and quality of inbound links,other factors affect the PageRank, such as the number of visits to the page and the search words that are used on the page.
To prevent sites from manipulating or spoofing PageRank, very little details are provided by Google as to what factors actually affect it.