What's a Quality Link?
We explained you that it is of primary importance to have sites which link to your's. If your little brother, who has just published the first pages of his site, put a link towards your site, unfortunately that will not undoubtedly have little influence on the popularity of your site. On the contrary, if Yahoo! decides to put a link towards your site from their homepage, then your site will quickly be propelled at the top of the results!
In fact, a quality link is a link from a page having a strong popularity, i.e. good PageRank in the case of Google.
This said, there are two other significant criteria: the number of links and the context of the linking page. More precisely, the format of the link must comply with some rules to be optimal.
