Black Hat SEO is the process of using unethical, unapproved, or even illegal tactics to grow a site's traffic and rankings through search engine optimization (SEO). Over the long-term, black hat SEO practices can seriously hurt a site's traffic and credibility, and even get you banned from search engines.
Black hat SEO refers to a set of practices that are used to increases a site or page's rank in search engines through means that violate the search engines' terms of service. The term "black hat" originated in Western movies to distinguish the "bad guys" from the "good guys," who wore white hats (see white hat SEO). Recently, it's used more commonly to describe computer hackers, virus creators, and those who perform unethical actions with computers
Basically it's a slang name for all techniques that go against search engines' rules. They are usually used to improve rankings quickly, but getting out of penalties doesn't worth it.
Black hat SEO is a practice against search engine guidelines, used to get a site ranking higher in search results. These unethical tactics don't solve for the searcher and often end in a penalty from search engines. Black hat techniques include keyword stuffing, cloaking, and using private link networks.