What are black hat SEO techniques?

[singhaniaInt]

A black hat does not follow Guideline of a search engine. In search engine optimization (SEO) terminology, 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 seo techniques are

Doorway pages
Keyword Stuffing
Link Farming
Spam url
Tiny URL

[davidweb09]

SEO Techniques which doesn't follow Google Guidelines are treated as Black Hat. For Ex. Duplicate Content, Low Quality Backlinks.

[deepak-sh]

Black Hat SEO is a search engine guide that is used to get higher site rankings in search results. These unethical strategies do not solve the problems of search engines, and are often penalized by search engines. Black hat technology includes keyword padding, hiding real content and using a dedicated link network.