Any "trick" or practice you are having to go out of your way in order to achieve a higher ranking in the search engines is probably a spam tactic. When in doubt, ask yourself "will this practice benefit my users in anyway?" If the answer is no, then you're probably spamming. Example: Hidden text does not help your users in ANY way, they can't even see it! Usually the driving force behind such a practice is the hopes of ranking high in the search engines.
The following are considered spam tactics: