Black Hat techniques are techniques search engines frown upon to get web pages higher in the rankings. Some common black hat techniques are doorway pages, keyword stuffing, and putting invisible text on the web page. Doorway pages are usually poorly designed pages with nothing but links back to the main page that is meant only for search engines. Keyword stuffing is deliberately loading your web page with keywords, and invisible text is self explanatory. Some of these no longer work, and others will work for a while before the search engine decides not to index the pages that included them. It is best to avoid black hat SEO techniques.