Black Hat SEO is an unethical methods of gaining traffic, such as using the most popular and successful websites as mechanisms to divert traffic. e.g. posting videos on YouTube with your websites link within the video or in the description or title. Having 1,000s of people on twitter following you and then advertising your website every so often, creating fake social networking or adult finder accounts with a hot lady in the profile promising XYZ if they visit GVids, some people even go as far as running scripts to spam forums with links or whatnot.
I can remember a while back I downloaded a shitload of porn videos, uploaded the videos to all the major streaming websites with a link to my website showing at the bottom of the video throughout, it converted into 1000s of unique visits per day for like a month, when it died down I was left with around 100-200 visits per day. Which is good for a website that was getting 10 visits per day previously.