A basic tenet of all the search engine guidelines is to show the same content to the engine's crawlers that you'd show to an ordinary visitor. This means, among other things, not to hide text in the html code of your website that a normal visitor can't see.
When this guideline is broken, the engines call it "cloaking" and take action to prevent these pages from ranking in their results. Cloaking can be accomplished in any number of ways and for a variety of reasons, both positive and negative. In some cases, the engines may let practices that are technically "cloaking" pass, as they're done for positive user experience reasons. For more on the subject of cloaking and the levels of risk associated with various tactics and intents, see this post, White Hat Cloaking, from Rand Fishkin.
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