Mirror websites are simply sites that all have the same content but use a different URL. This is a technique that is being used less as the search engines are getting better at penalizing this type of duplication.
Another clever black hat SEO trick is URL redirection. No matter what you click on the user is taken to a site that they did not wish to go for. This can backfire as often all this does is make people really angry, unless what you have to offer them is truly captivating.
Yet another popular technique for fooling the search engines is called cloaking. Cloaking refers to any of several methods that are use to offer up a different page then will be seen by humans to the search-engine spiders. Yet another tricky technique is which is a form of code swapping. Once a page has reached a high ranking in Google the code in the URL is then changed so that the surfer is taken to a totally different page.
Other forms of spamdexing are Google bombing and Google washing. Google bombing is the practice of deliberately modifying HTML pages to increase the chance of their being placed close to the beginning of search engine results, or to influence the category to which the page is assigned in a misleading or dishonest manner. Googlewashing is used to change a person's perception of a term. SEO experts sometimes take advantage of Google bombs and washes by linking to them so that ratings can be improved.
