For the purpose of better search results the concept of link popularity was developed. This means that a Web page is generally more important, if many other Web pages link to it. Google Page Rank is a numeric value ranging from 1-10 that works as an indicator of the general importance Google has assigned to the individual Web page. The Google Toolbar displays the Page Rank and several other features useful for SEO. Google figures out that when one page links to another page, it is effectively casting a vote for the other page. The more votes that are cast for a page, the more important the page must be. Also, the importance of the page that is casting the vote determines how important the vote itself is.
When a site votes via a link for another site all the pages in its internal link structure and the Page Rank are affected. Outbound links are links that start from your site and lead to another one. Outbound links are a drain on a site's total PageRank. Use the rel="nofollow" tag in the links to Web sites that you do not trust, you think they maybe using spamming techniques, viruses or spyware or not relevant to your web page. Google algorithm has become much more elegant and advanced, devaluing staggering amount of links that shouldn't count, and placing more emphasis on trusted links. And the trust and juice given by those links is then verified by elements like user data, domain age, and other relatively hard-to-spoof factors.