nice SEO FAQ

One question that comes up again and again, is what is cloaking. And how is it related to the SEO field. As you can guess cloaking is about hiding something, and in this case it is about hiding (or possibly showing something different) to the search engines. It is possible to have a different webpages displayed depending on the type of visitor, if you know your visitor is from Germany, it might be better to show them the German version of your website as opposed to the French version. Finding out information about the user could be done on a USER-AGENT basis or IP basis for example. 

In the previous example, different versions of a webpage were displayed depend on a users presented information (i.e. an IP address), but it does not only related to visitors. Cloaking can be done for search engines , so that search engines spiders will like your content even more. And this is considered cloaking. You can probably understand why search engines depend on seeing what a normal user sees. If you decide to show google a certain page, with higher keyword density (which might make it unreadable) and with other optimised factors, which when seen by a real human being would be unusable, this tactic you are doing is considered cloaking. Search engines like google, yahoo and MSN all take a poor view at cloakers.

Cloaking is considered blackhat SEO, and is a very dangerous tactic to use. If caught you risk getting your website banned from the search engines.