Black hat SEO is search engine optimization (SEO) that uses less than legitimate techniques to achieve results. There are two kinds of search engine optimization, white hat and black hat. White hat SEO is where rules are followed and approved techniques are used to improve a website search ranking. Black hat is where someone cheats to get the same result.
Many techniques that are now considered black hat SEO used to be widely used and perfectly legitimate. Some people abused these methods and caused problems for others on the Internet. This meant the methods were quickly disapproved of by search engines and frowned upon by the SEO community.
While many black hat SEO techniques work well, they most often only cause short-term gains. Search engines have become very sophisticated and can detect when people are trying to cheat them, or use unapproved techniques to achieve results. This often means ranking penalties, and even the website being removed from the search engine altogether.
Techniques such as keyword stuffing used to be prevalent among the SEO community. This is a practice where a given keyword was given on which to rank a site. The site would then use that word as many times as possible in its text.
For example, a website that sold computer memory would choose the keyword "computer memory," as that is what their potential customers are likely to use in a search. The SEO person would then craft the site copy so it mentioned those words as often as possible. This was so the search engine saw how many times those words were mentioned and calculated it was a good site to put high in the results. Unfortunately some people took this too far, often creating websites that were unreadable, or at least unpleasant to read.
Using invisible text was another trick used by the SEO community until it became frowned upon. A list of keywords would be placed on a web page in the same color as the background for the search engines to find. This did not affect the website visitor, but did skew the search engine results, and was regarded as cheating.
There are many other black hat SEO techniques that are used that a website visitor would never be aware of. While the case for the methods being black hat is mainly in the SEO community's interest to make, the user has an interest too. He depends on the search engine giving him the results he is looking for without wasting his time on sites with no relevance. It is in the search engine's best interest, therefore, to keep the SEO community in line and the playing field as level as possible.