Cloaking is a type of black hat search Engine optimization (SEO) method in which the content offered to the search engine spider is different to that existing to the user's browser. The reason of cloaking is to mislead Search Engines so they display the page when it would not otherwise be displayed.

So cloaking is not measured necessary by proponents of that method. This technique is essentially to show different content to the user and the bot that crawls the website, so that you can manipulate what actually indexes. The "cloaking" is a penalized by search engines on the web. Some search engines like Google, offer the possibility for users to report any site that uses these techniques. Other fraudulent tricks that are used to help rank pages are Doorway pages, hidden text, duplicate pages, hidden links, spam in weblogs and books of signatures, the abuse of keywords and the purchase and sale of links

It is a method of positioning in Internet search engines, usually penalized by the major search engines.
Basically consists of "show" to the search engines (like Google or Yahoo) versions of the pages other than the actual. This version is optimized for a page to appear first in searches for certain words (which would not be able to get the actual page, which is what visitors are really common).