Dear Aghor, All technique which is not allowed by search engine link keyword stuffing, clocking, doorway page etc, But we do these activities to improve ranking of keyword is know as Black hat seo. To get more information you can also visit:- http://seo4rank.blogspot.com/2010/08/what-is-black-hat-seo.html
Hi Aghor, Activities like keyword stuffing, clocking, doorway page and other are counted as black hat SEO. These activities mainly used for improving SERP results.... If these activities were notice by any SE then you have to bare a heavy penalty...
"Black Hat" techniques are unethical methods that some Web site owners use to get their site listed on search engines. Here's a list of three common "Black Hat" techniques you should avoid since they could get your site banned. Keyword stuffing: Keyword stuffing is the overuse of keywords in a piece of content. Generally, this is repeating the same keywords over and over just to achieve a higher search engine ranking. Invisible text: Quite a few cheap sites use this tactic, which involves making keywords to be the same color as the background, so readers can't see the massive amounts of keywords, but search engine spiders can. Doorway pages: These are pages that regular visitors cannot see, but search engine spiders can. They are used to trick the search engines so that the site gets a higher ranking. Not only do they go against the rules, but they also hurt the visitor's experience.
It originates from Black Hat hackers, Black Hat SEO is pretty much "hacking" Google or others to make believe your website is more important than another, some techniques can be legal and others not.
Black hat SEO are techniques that are not following the search engine rules and regulation. Some of the tactics use in black hat are: cloaking, doorway pages, keyword stuffing, link farm etc. Better do white hat technique if you want your site not to be banned by search engine.