Referrals ( Back links )
The web is one big bibiliography
The genius of page rank ( Google's Initial algorythm ) is that web sites sites frequently link to one another there by giving each other credibility. If I make a claim on my web site and attribute another web site, then I am essentially saying: "I believe that the other web site is a credible and authoritative source for the claim I have just made." The more CREDIBLE and RELEVANT back links I have the more of an authority I am in the eyes of the search Engines.