Not everyone knows factors that have the greatest impact for a truly high-quality backlinks, which can significantly improve your site position in SERPs. Analysis, testing and attracting of backliks play an important role in SEO, as Google rating is calculated on the base of PR of pages referred to your site.
When it comes to determining the place of the site in search results, incoming links are considered as a kind of recommendation given to your site by other resources. The quality (PR) of sites, referred to yours, predetermines its authority. So it’s necessary to choose relevant (of the same topic) resources.