Answer

The term "black hat" refers to search engine optimisation techniques and tactics that are disapproved by search engines and deemed manipulative. Such tactics are considered unethical and will put your site at risk of being banned or penalised. Such techniques may range from forum spamming to illegal techniques such as server hacking and cross-site scripting. You can see Google's SEO Guidelines here: http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=35291