Black hat SEO are the Search Engine optimization methods which are considered as unethical by search engines.

These methods can lead to banning of the website from the search engine results.

These are used for manipulating the ranking of the website on search engine results. Some of the example of  Black hat SEO are:

(a) Keyword stuffing:
Excessive use of keywords (keyword repeatedly used) on a webpage either as a text or
as hidden HTML elements in Page title, or Meta tags.
 
(b)Cloaking: Presenting different content to search engines than what you display to users.

(c)Purchasing links and participating in mass link exchage program.