‘Negative SEO’ means anything malicious someone does to harm your site’s rankings. If someone really attacks your link profile and if it going to be effective you’ll see unexplained. The Impact of an attack is often temporary. People many times use methods that get quickly removed or discounted and when the link go away, the problem always goes away. It is easy to be so aggressive that Google spots the attack and devalues the links. If someone really is out to get you and wants to spend time and money, there is no doubt, they can do a lot of damage . The SEO problems for websites in 2012 in 99% of cases are self-inflicted.
Read the full article here:
Were You Hit by Negative SEO? | SEOmoz