Cloaking? This ain't Star Trek, Jim!

There was a time when the word cloaking was associated with a Klingon Battle Cruiser disappearing from the Enterprise's screens and proceding to unconvincingly throw Captain Kirk out of his chair.

These days, however, the term cloaking is most commonly associated with a black-hat SEO Tactic whereby a web server displays different versions of a site to humans and search engines.

How is it done?

Basically, any visitor to any given website is identified for what they actually are. It's as though your computer were saying: "My human wants to look at this web page, now show it to me please."
At this point, the web server on which the website in question actually lives, thinks something like:
"Ah-Hah! A human! That means they have to see this version of the page they're asking for."

It then proceeds to show its human visitor the web page which simply says "Buy me!". If, on the other hand, a visitor identifies itself as Google or Yahoo! for instance, the web server automatically digs out and displays an information-rich version of the page in question, so as to feed this search engine with the high-quality content it craves.

The problem with this technique, is that Google makes it perfectly clear that it wants to be shown exactly what its users are shown, no more... no less. After all, if it were shown a completely different web page than a human visitor, how could it possibly assess a website's true relevance to any given search by a user? And that, you might say, is exactly the point.