A website’s page rank, is how ‘important’ it is on the web.  It is essentially a popularity meter.  Popularity or importance is determined by the amount of links relating to the page there are, there are four different types.  Inbound, links from other pages to yours.  Outbound, links from your page to others.  Dangling, links to a page which has no links to others.  Deep, links to a specific page, usually bypassing the homepage.  The page rank algorithm takes the probability of a ‘random surfer’ becoming bored and requesting another ‘random page’ (otherwise known as the dampening factor) away from 1 and divides this number by the number of pages in the system, adding it to the dampening factor multiplied by the page rank of a linked page divided by the number of outbound links on that linked page.  Adding on this last section for every other page linked to from the original page.  Google uses this algorithm to assist intentional surfers in finding the best websites to suit their needs.  One of the problems with this popularity algorithm is that it is easily manipulated and can give false values, hence the frequent recalculating of page ranks.