Black hat SEO is the unethical form of SEO. It refers to the use of techniques which are not acceptable to search engines to improve the rankings of the site. These techniques aim to fool search engines to give better rankings to their pages. The techniques used in black hat SEO will not have any benefits to the users. Black hat SEOs can be spammers but employ other kinds of techniques too. Examples of some other kinds of black hat SEO techniques are: link farms, hidden text, scraping, doorway pages, cloaking, comment spam, paid links/content+, keyword stuffing, etc.
Despite being unacceptable to the internet community, black hat SEO techniques are used because they work until the search engine finds out about them and lower awareness about the fact that search engines will ban their sites once they find out about the use of such techniques.