The PageRank algorithm is used to designate every aspect of a set of hyperlinked documents with a numerical weighting. It is used by the Google search engine to estimate the relative importance of a web page according to this weighting. The system uses probability distribution to determine the odds that a person randomly clicking on links will arrive at any given page. Following this, each web page is given a ranking of 0-10 according to its relevance to a search. The PageRank is calculated by taking into consideration the number of inbound links, and the PageRank of the pages supplying these links. This means therefore that if a webpage is linked to others that have a high ranking, then it too will receive a high rank. 

Due to the nature of the PageRank system, it is susceptible to manipulation and has been exploited so that certain pages are given a false, exaggerated ranking. In these cases, only Goggle has access to the genuine PageRank. However, much research has been conducted into methods of avoiding links from documents with a false PageRank to try and iron out the bugs in this system and from 2007 Google has actively penalized schemes which try to increase rankings artificially.