Google has devised a system that looks rates a website on importance, this is called PageRank or PR. The PageRank system was brought into play in the early 20th century to give websites an authority rank. In simple terms PageRank is calculated on how many important links you have pointing to your website.
A lot of SEO experts disregard PageRank as it in know way helps with SEO and does not rank you higher than any other site.
In the past I have worked on a tourism website, which I had to get to number one for a certain term, after a few months work I managed to knock the BBC News website off the top spot of Google, the BBC News website at the time had a PageRank (PR) of nine while the tourism site I was working on had a PR of five, so there is proof that PR does not effect rankings.
It has been rumoured that PageRank will be removed from Google due to PR being manipulated through link farms and buying.