Links were the first major “Off-The-Page” ranking factor used by search engines. Google wasn’t the first search engine to count links as “votes,” but it was the first search engine to rely heavily on link analysis (or the Link Graph) as a way to improve relevancy.

Despite the chatter around other signals, links remain the most important external signal for search rankings. But as you’ll find, some links are more valuable than others.

Lq: Link Quality
If you were sick, which would you trust more? The advice from five doctors or from fifty random people who offered their advice as you walked down the street?

Unless you’ve had a really bad experience with doctors, you’d probably trust the advice from the doctors. Even though you’re getting fewer opinions, you’re getting those opinions from experts. The quality of their opinions carries more weight.

It works the same way with search engines. They’ll count all the links pointing at websites (except those blocked using nofollow or other methods), but they don’t count them all equally. They give more weight to the links that are considered to be of better quality.

What’s a quality link? It’s one of those “you’ll know it when you see it” types of things in many cases. But a link from any large, respectable site is going to be higher on the quality scale than a link you might get from commenting on a blog. In addition, links from those in your “neighborhood”, sites that are topically relevant to your site, may also count more.

Source- http://searchengineland.com/guide/seo/link-building-ranking-search-engines