here ya go, it’s all in here: http://en.wikipedia.org/wiki/Cloaking
Quote:
The decision to cloak or not is often based upon the HTTP referrer, the user agent or the visitor’s IP; but more advanced techniques can be also based upon the client’s behaviour analysis after a few page requests: the raw quantity, the sorting of, and latency between subsequent HTTP requests sent to a website’s pages, plus the presence of a check for robots.txt file, are some of the parameters in which search engines spiders differ heavily from a natural user behaviour. The referrer tells the URL of the page on which a user clicked a link to get to the page. Some cloakers will give the fake page to anyone who comes from a web directory website, since directory editors will usually examine sites by clicking on links that appear on a directory web page. Other cloakers give the fake page to everyone except those coming from a major search engine; this makes it harder to detect cloaking, while not costing them many visitors, since most people find websites by using a search engine.