What is a Backlink?

Not all backlinks are created equally here are some features that contribute to a backlink’s quality and capability 

Relevance
As people are more likely to click relevant backlinks therefore search engines like Google places more value on relevant backlinks for example if the plumber has backlinks from two pages, one about dogs and one about installing boilers, chances are the latter is more likely to be clicked on.

This is also applicable at domain level.  Readers of plumbing.com are more likely to click on a link to a plumber’s website than readers of dogs.com 

Authority
Page level authority is connected with organic traffic as backlinks from strong web pages usually transfer more authority than those from weak ones. 
It’s mentioned in Google’s original patent that authority is split equally among all outbound links on a webpage. So if you have backlinks from two pages and the number of outbound links is different, one has more than the other, then all else equal the link from the page with fewer outbound links transfers more authority. 

Google has a fair number of patents describing a number of various methods for assigning value to backlinks. 

Moreover internal* backlinks also contribute to a page’s authority.
*If one page links to another on the same website, that’s known as an internal backlink.

Traffic
It is obvious that backlinks from high-traffic pages will usually send you more referral traffic than those from low-traffic pages. The question is whether the backlinks from high-traffic pages have the same effect than those from low-traffic pages.
Actually there is a small but clear correlation among rankings and backlinks from pages with organic search traffic. However the total number of backlinks from particular websites (domains) and page levels authority seems more important. 

Placements
Some links on web pages likely pass more authority than others because people are more likely to click prominently-placed links. 

Bill Slawski talks about this in his analysis of Google’s updated “reasonable surfer” patent.

“If a link is in the main content area of a page, uses a font and color that might make it stand out, and uses text that may make it something likely that someone might click upon it, then it 

could pass along a fair amount of PageRank. On the other hand, if it combines features that make it less likely to be clicked upon, such as being in the footer of a page, in the same color text as the rest of the text on that page, and the same font type, and uses anchor text that doesn’t interest people, it may not pass along a lot of PageRank.”     

Consider this while pursuing links. If your link will probably end up in the site’s footer, or along with fifty other sites in the sidebar, then put your effort into other opportunities.

Follow vs. nofollow
Although nofollowed backlinks can influence the linked page’s ranking but they usually don’t.

Because link building requires time and effort, it’s good to get do-follow links. Just don’t kick up a fuss if you get a nofollowed link. It may still have some SEO value.

Anchor text
Anchor text are the clickable words that form a backlink.

It’s mentioned in Google’s original patent that anchor text influences ranking. Google employs a number of techniques to improve search quality including page rank, anchor text, and proximity information. That said, when we scrutinized the relationship between anchor text and rankings across 384,614 pages, the correlations were weak.

So while anchor text does matter, it’s not as essentials as other things.