It wasn’t too long ago that shady SEOs discovered they could engage in negative SEO tactics and kill their competitors in the search engine rankings. But recent changes at Google have weighed in favor of the innocent victim.
Matt Cutts explains negative SEO briefly in a recent Google+ post:
If you received a message yesterday about unnatural links to your site, don’t panic. In the past, these messages were sent when we took action on a site as a whole. Yesterday, we took another step towards more transparency and began sending messages when we distrust some individual links to a site. While it’s possible for this to indicate potential spammy activity by the site, it can also have innocent reasons. For example, we may take this kind of targeted action to distrust hacked links pointing to an innocent site. The innocent site will get the message as we move towards more transparency, but it’s not necessarily something that you automatically need to worry about.
If you believe that you have been the target of a negative SEO campaign and it has affected your search engine rankings, submit a site review from within Webmaster Tools. Give as much information as you can to Google. You can let Google know that you don’t trust certain links and that they shouldn’t either. This will go a long way to weigh in your favor.