Cloaking is part of black hat technique. It is a technique used by some websites to deliver one page to a search engine for indexing while serving an entirely different page to everyone else.
The term "cloaking" can also be used for other techniques which are not black hat. For example, if you are using your affiliate link in product review, you might want to "cloak" your link by aliasing it to an internal folder which contains a PHP redirect to the partner site. An example of this technique is here > http://clickbump.com/how-to-cloak-affiliate-links-in-wordpress-with-clickbump-seo/