It is the illegal or unwanted way of using SEO strategies.It is banned By Google after Penguin update.Keywords Stuffing,link Exchange and Clocking are some terms which are comes in Black Hat.

Black hat SEO techniques are the process which does not maintain the search engine guidelines and white hat SEO is the process which follows the search engine guidelines. In terms of Google, white hat SEO is the best process to maintain a good SEO track.

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Black Hat Search engine optimization when integrated into a website creates a poor user friendly experience.