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Search engines are not infallible. We have all, at some point in our searching lives, found a link on the web that seemed interesting or helpful but when clicked were disappointed to find that the destination page was rubbish. Not only is it frustrating but a complete waste of time.
Search engines such as Google, Yahoo! and MSN wants to ensure they are showing relevant results to their end users. For those who employ successful “Black hat” or “Unethical” techniques to get their sites to show up in the search results pages, more often than not, will not give you the information you were looking for.
On the other hand, Black Hat SEO includes efforts like redirecting search engine "spiders" to different pages than human visitors see, mass-posting "spam" comments (on blogs, forums, articles), or putting lists of keywords at the end of each page in very small fonts. Black hat methods may actually reduce the quality of search engine results.
Duplicate Sites
While this is not often used, it was popular for a very long time and deserves to be mentioned. When affiliate programs were first gaining in popularity, webmasters would create several copies of the same sales page in hopes that quantity over quality would prevail and they would make a sale from one of their many websites selling a product. With the advancement of search engines, they are now able to find excessive duplicate content and rank it accordingly.
Black hats do this because it's easier to rank for a 4,5 or even 6 keyword search that has no direct competition than it is to rank for a search phrase that has hundreds of thousands or millions of results.
This is the easiest and the most common spamming method. It simply means to set text in the same colour as the background, use a tiny font size or hide it in HTML code sections. For a real person looking at the website nothing will change, but for a search engine crawler, it will make a difference. The code will have more content and so it will find a higher keyword density.
A website can be penalised when its overloaded with a specific sentence/keyphrase in order to make a search engine think the content is more relevant than others. This technique can be used in combination with hidden text too. Most of the search engines now have mechanisms in place to avoid the abuse of keywords.
Probably the easiest way to get blacklisted and lose even genuinely earned rankings, doorway pages are appended to a website, giving no value to a visitor. The page exists solely to attract search engines to the targeted keyword or phrase. These pages are usually mass-produced and posted automatically by software.
In conclusion, using any techniques considered spammy and unethical by the search engines could damage your site. Many of these techniques might work in the short-term but, unless you want your site flagged or banned, do avoid them. On the other hand, black hat SEO raises questions when it comes to the general credibility of your online business and website.
Read about search engine submissions, and also read about submit url and manual directory submission