What is the importance of follow vs nofollow links for SEO?

[hossen15-9477]

An important point about the nofollow link attribute that I have not seen mentioned yet in these answers is why you would want to use it. So here is a very brief history.

Once upon a time, there were lots of webpages with places to post comments. Anyone could post a comment and they could say whatever they wanted. It didn't matter if the comment had anything to do with the main point of the article or blog post. They could also post links. Links are good for SEO. There are lots of people around the world who would gladly post links to websites in the comments area of sites all over the web that point to your website for just $5 a link. That's some very inexpensive link juice and a great way to boost the domain authority of your website. 

This tactic created what became known in the industry as "comment spam" (some also called it link spam). Website owners could not keep up with the amount of garbage appearing on their webpages.   And Google hated that this technique was working to game their ranking algorithm.

To combat this black hat technique, they instituted the rel="nofollow" link attribute. Now website owners had a way to remove the incentive from black hat SEO's to engage in this tactic.  The attribute was a simple way for the site owner to say "I can't vouch for the integrity of this link since I didn't put it there. Google, I would not recommend passing any link love through this link."

And lo, it came to pass that comment spam would be entered into the museum of black hat SEO tactics that no longer work.