Since Google’s Penguin 1.0 update launched in April 2012 (and a recent update on Oct. 4), a painful reality had dawned on us: inbound links could negatively impact on our Google rankings.
Prior to this, the worst an inbound link could do to your site was nothing at all. Google suddenly changed the rules, and we had to look out for potentially “toxic” links pointing to our sites. Which can be scary, as these links are often out of our control.
Due to the free-for-all nature of link building at the time, many sites were accumulating toxins faster than a chain-smoking alcoholic. In a few short weeks, the talk switched from building links to cleaning up link profiles. It was time to detox and swear off indiscriminate link building.
Read more: http://www.searchenginejournal.com/approved-spot-bad-links-deciding-whether-take-action/72819/