The PageRank algorithm is the most popular method to rank / weight websites. The algorithm was developed and published by Sergey Brin and Larry Page at the Stanford university end of the 90s. PageRank can be understood as the importance of a website. The ranking algorithm is a function of on- and off-page factors. On-page factors are for example the title, description, headings and plain text. An off-page factor apart from PageRank is the anchor text (of incoming links). Neither the content nor the URL plays a role (such parameters are called off-page factors). Moreover, there is no difference between internal and external links.

The citation (link) graph of the web is an important resource that has largely gone unused in existing web search engines. Google created maps containing as many as 518 million of these hyperlinks, a significant sample of the total. These maps allow rapid calculation of a web page's "PageRank", an objective measure of its citation importance that corresponds well with people's subjective idea of importance. Because of this correspondence, PageRank is an excellent way to prioritize the results of web keyword searches. For most popular subjects, a simple text matching search that is restricted to web page titles performs admirably when PageRank prioritizes the results (demo available at google.stanford.edu).

Assume page A has pages T1...Tn which point to it (i.e., are citations). The parameter d is a damping factor which can be set between 0 and 1. We usually set d to 0.85. There are more details about d in the next section. Also C(A) is defined as the number of links going out of page A. The PageRank of a page A is given as follows:

PR(A) = (1-d) + d (PR(T1)/C(T1) + ... + PR(Tn)/C(Tn))


Note that the PageRanks form a probability distribution over web pages, so the sum of all web pages' PageRanks will be one.
 

PageRank or PR(A) can be calculated using a simple iterative algorithm, and corresponds to the principal eigenvector of the normalized link matrix of the web.