Negative SEO Techniques
In the midst of a cut-throat market of competitors and websites, all vying for the number one position, someone is going to be at the top while leaving the others at the bottom. The whole concept of negative SEO is to demote the site rankings of your competitors, allowing more room for your own site to shine.
In the same way that Search Engine Optimization is used to promote a site into higher search engine traffic using various techniques, negative SEO is the same, only with the opposite effect in mind. The following are some of the most widely used negative SEO techniques out there.
Firstly and most obviously, if you manage to find "dirt" on your competition, you can use such evidence to get them in trouble with the search engines. Most search engines (Google included) have a policy against keyword cramming and link farming, so if you manage to find evidence that a competitor is engaging in one of these practices, you can notify the search engine and potentially have the site flagged or taken out of the search results.
Secondly, hosting providers and forums are required to take down sites that have posted content that infringes or has made a copyright violation if they receive a DMCA complaint. Although if the competitor is not really in violation of a copyright, they may end up filing a lawsuit against you, so be absolutely sure that they are indeed in violation before attempting this.
The next method seems counterproductive, but it works. Let's say there is a page out there with negative information against your site, like a bad review. If you copy this information and put a copy on a page on your own website, the engines will favor your page and give the original page lesser ranking. This will only work however if your own site is already in good favor with the search engines.
These techniques, also known as "Google Bowling", are becoming more common as time goes by and as competition gets stiffer. The techniques listed here are by no means exhaustive, but do give a solid example of some common methods.