Pagerank - A logarithmic scale based on link equity which estimates the importance of web documents. Since PageRank is widely bartered Google's relevancy algorithms had to move away from relying on PageRank and place more emphasis on trusted links via algorithms such as TrustRank. The PageRank formula is: PR(A) = (1-d) + d (PR(T1)/C(T1) + ... + PR(Tn)/C(Tn)) PR= PageRank d= dampening factor (~0.85) c = number of links on the page PR(T1)/C(T1) = PageRank of page 1 divided by the total number of links on page 1, (transferred PageRank) In text: for any given page A the PageRank PR(A) is equal to the sum of the parsed partial PageRank given from each page pointing at it multiplied by the dampening factor plus one minus the dampening factor.
see definition: Page Rank Devised by Google, it measures not only how many links point to a website, but the "quality" of the sites providing the links.
PR is useless and is not a factor in googles algorythm... you should be asking what is SEO. Do some research on something worthwhile.