Google uses PageRank as one measure of the popularity of a website page. However, the concept of link popularity and pagerank has a weakness, that is opening the possibility for manipulation.
In simple terms it can be said that PageRank is the result of the calculation quantity and quality of inbound links pointing kesebuah website pages. a page will be pouler and dibandroli high pagerank if you have a lot of inbound links and the quality with the assumption that the number of other web pages that provide a reference (in the form of links) kesebuah page of the site show the page has content or content that is useful.
If you have a new web site or its contents mediocre, of course you can not expect many other site owners will voluntarily provide a reference link to your site.
Received inbound links from other sites can not voluntarily, what a way to buy.
This is where the manipulation. To increase the page rank of their websites, the web master (website owners) many prefer the shortcut, which is to buy links from other websites.
Indeed, this is not natural, but in many cases able to boost the pagerank of a site obtained although the link is a link prepaid (Paid Link). This is what fosters the practice of buying and selling links among online businesses.