Google's PR (PageRank) has been the factor which set google apart from the other search engines. PageRank plays an important part in how your website will rank on a particular search engine. Basically every time a website links to your website it casts a vote for your site. The higher the PageRank of the site that is linking to you the more weight the Vote carries. In Effect each site will pass on some of its PageRank also know as link juice. Google however recommends that sites that pay for a link have the nofollow attribute in the hyperlink. Adding the NOFOLLOW tag will prevent the link from passing on Page Rank.