SEO Guide - Part 4 : Back-Links - Web 4 Panama
A very important ranking factor is the number of linking pages. It used to be measured solely by quantity. This was soon abused of course, and many webmasters were able to boost their websites in the SERP´s just by buying them. Google turned on this eventually and now assess the rank or ´authority´ of the back-linking page and domain, and also the topical relevance of the linking page and domain to the landing page and its domain.