The main ranking factor with search engines (especially Google) has always been Back-Links. A simple link from another persons site counts as a "vote" to say your site has something on it that is worth sharing. The SEO power of each link is different and depends on lots of different factors, some we know for sure.... others we dont. What we do know is that a link from a high Page-Rank site is wirth more than a low Page-Rank link. We also know that a page with less links on it gives each link a higher power. Another thing we can be sure of is that the "Title" text of a link (the text that pops up when you roll over the link) is how the search engines know what is inside the link, as well as the actual text used that you can see on the page.
