Black Hat SEO is the use of dubious optimisation techniques in an attempt to manipulate the search engine's algorithms to rank a website or webpage higher in the search results.
There are numerous tactics that dishonest digital marketers have employed over the years in an attempt to improve their search presence, a few of the more popular methods being:
Black hat techniques were once very successful and most SEOs who have been in the game a long time will have at one point or other used techniques now deemed black hat. With several incremental advances in the Google algorithm, courtesy mainly of Panda and Penguin updates, black hat techniques either no longer work, are on the way out or have already been penalised.
While the SEOniverse would never in a million years suggest using any black hat tactics for your own short-term ranking gains, monitoring the successes an failures of other Black Hat sites has the advantage of giving insight into when the search engine algorithms change. http://www.blackhatworld.com/
Black Hat World is a digital marketing community marketplace that's all about making money online using blac khat methods. Monitoring the forums here when the search engine algorithms are in flux can give you a fantastic idea of the types of black hat tactics that are being penalised and inform you if your site is at risk of a penalty.