Black hat SEO refers to specific techniques and methods in optimizing a web site for search engines to read them. Black hat is the opposite of white hat SEO.
Black hat essentially refers to all underhanded and deceptive means of driving traffic to your site. These aren't illegal, but are meant to trick search engines into indexing the site different than it really is.
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An example of a black hat SEO tactic is to create a landing page that a visitor will never see, but exists to attract search engines and tricking them into showing your page. The result is that visitors will land on another page that is totally unrelated to the link they clicked on.
Risks of Black Hat SEO
Traffic gained by this method is irritating to visitors and dangerous to site owners. Expect to be either penalized or banned by the search engines if you (or your agency) are promoting a site by these techniques. Hiring the best SEO agency is important. Black hat techniques can generate penalties for your site.