Black hat SEO is a set of techniques to intentionally try to manipulate search engines' rankings for their targeted keywords to the benefit of website owners. Techniques are generally considered to be black hat if they violate search engines' webmaster guidelines or terms of service. Typical black hat techniques include all kinds of link spam, anchor text manipulation, keyword stuffing, cloaking, article spinning, using scraped or stolen content, and more.
Search engines are always fighting an ongoing war with black hat SEO practitioners. While search engines continue to improve their detection algorithms and make techniques ineffective, black hat can often be effective in the short term while search engines scramble to combat new techniques.
Black hat SEO is generally considered to be an extremely risky and short-term strategy. By staying within search engines' webmaster guidelines, your site is assured a long-term strategy that will steer clear of manual and algorithmic penalties from search engines.