What is Black Hat Seo ? And how many techniques are used in it ?

Black hat seo is technique in which we use duplicate issues like content,photos, video,hidden links,keyword stuffing doorway pages and many other.It is not good for our site because when Google starts its crawling it considered the site is full of duplicate issues and he penalized the site minimum +30 Google page ranking hidden link keyword stuffing doorway pages incorrect keyword link farming mirror side Note:- For Best seo this technique is not good avoid this technique..

Black Hat techniques are nothing but attempt to game the system by finding its flaws, there are a lot of this type of techniques and tricks but majority of which don't work now-a-days as the search engines are getting more and more intellegent.

And moreover it is very unadvisable to deal with any of the Black hat techniques, as it may give you a short time boost of your ranking but will eventually penalize your website beyond recovery.