Black hat SEO

1. Keyword Stuffing – This is the most obvious technique, and one generally employed by laymen and SEO practitioners that are not very well versed in proper technique. This technique involves inserting long lists of keywords directly into web pages. Proper keyword placement is actually much harder and needs to work with your natural content rather than against it. Sites that utilise keyword stuffing are obvious since the lists of keywords begin to take over the layout of the site.
2. Invisible Text – Use of invisible text (i.e. white text on white background or black text on a black background etc.) can be used to hide key phrases so that they don't take over the design of the site, usually used in tandem with keyword stuffing. This technique breaks search engine rules because a search engine spider is supposed to read what a real user can see.
3. Gateway/Shadow or Doorway pages – These are pages that are heavily pitched toward search engines, but are not visible to your users. Around 8 years ago, this was the main method of generating great positions within the search engines, the situation back then was that you're either employing shadow pages, or you're not in the search engines. Since then, search engine software has been rewritten to spot these fake pages, and so they don't have the power that they used to, indeed they break search engine rules and could get you de-listed. Many opportunistic search engine companies will offer unbelievable deals based upon this technique, based upon the fact that the buyer won't know that it's wrong – now you know!
4. Google Bomb/Googlewashing – It is possible to raise the ranking of a site or page within Google by getting people that own other sites to link to it with specific text. By having thousands of people link to a page using the same key phrase, that page eventually becomes number one for that phrase. This is actually a reasonable strategy and is still used by White Hat SEO practitioners today, however the correct way to do this is to link to that page using text that is actually relevant to that page. This means that SEO practitioners can no longer cash in on the biggest news item of the month e.g. we couldn't link to this page with the text "Recession comes to an end", however we could link to it using "Black hat SEO techniques" since it would be acceptable to Google.