black hat SEO refers to the use of aggressive SEO strategies, techniques and tactics that focus only on search engines and not a human audience, and usually does not obey search engines guidelines.

Black Hat" search engine optimization is customarily defined as techniques that are used to get higher search rankings in an unethical manner. These black hat SEO techniques usually include one or more of the following characteristics:

1.breaks search engine rules and regulations
2.creates a poor user experience directly because of the SEO techniques utilized on the Web site
3.unethically presents content in a different visual or non-visual way to search engine spiders and earch engine users.

If you work against Search Engine guidelines and use illegal method then its called Black Hat SEO.

It is harmfull for website,although it ranks your website fast.