The rel="nofollow" is an attribute you can set on an HTML <a> link tag, invented by Google, and adopted by others.
That links won't get any credit points when Google rank websites in the search results/
It removing the main incentive behind blog comment spammers robots.
Originally, the nofollow attribute appeared in the page-level meta tag, and instructed search engines not to follow (i.e., crawl) any outgoing links on the page.
For example:
<a rel="nofollow" href="http://www.sample.com/">Sample</a>
Here are some cases in which you might want to consider using nofollow:
Untrusted content
Paid links
Crawl prioritization
