Google has made it clear that links that are paid for or a link that was born out of some sort of financial relationship / compensation can be degraded, disallowed, or even grounds for punishment of a webmaster.
Google has recommended nofollowing such “paid” links so that it becomes clear to them that no pagerank is to flow through the link and the spiders are not to crawl the bad links. So how does one do this?
Quite easily in fact! To nofollow any particular link via HTML simply add the:
tag to your link. So for example, if you’re linking to a site a href=”link.html” simply add the rel=”nofollow” to change the link to a href=”link.html” rel=”nofollow”.
For more information on this visit Google’s official page here https://support.google.com/webmasters/answer/96569?hl=en