Search engines view incoming links as "votes" for the quality of your site by the websites that link to you.
So obviously, it's critical to obtain a good number of inbound links to your site.
There are a lot of very valuable link opportunities out there, if you only know where to look.
PageRank is Google's measure of your link popularity, on a scale of 0 to 10.
Limited links on the source page.
If the page title of the page linking to your site includes one of your keywords or is clearly related to the subject of your site, that makes it a relevant link.