Cloaking is a search engine optimization technique in which the content presented to the search engine spider is different from that presented to the users' browser; this is done by delivering content based on the IP addresses or the User-Agent HTTP header of whatever is requesting the page. The only legitimate uses for cloaking used to be for delivering content to users that search engines couldn't parse, like Macromedia Flash. However, cloaking is often used to try to trick search engines into giving the relevant site a higher ranking; it can also be used to trick search engine users into visiting a site based on the search engine description which site turns out to have substantially different - or even pornographic - content. For this reason some search engines threaten to ban sites using cloaking.
Cloaking is a form of doorway pages technique.
A similar technique is also used on the Open Directory Project web directory. It differs in several ways from search engine cloaking:
In more recent times several well known and well respected sites have taken up cloaking to deliver personalized content to their regular customers. In fact, many of the top 1000 sites - including household names like Amazon.com - actively cloak. None of these have been banned from search engines purely because of cloaking.