Google makes it clear that it disapproves of certain SEO tactics, such as hiding keywords with invisible text, or showing one page to Google's spider and another to actual human visitors.Methods that conform to what the search engines like are called White Hat SEO, and disapproved methods are called Black Hat SEO. There is a lot of controversy about whether Black Hat SEO is really "bad" in the ethical sense. White Hatters say that Black Hatters are unfairly trying to manipulate the SERPs. Black Hatters counter, "What constitutes 'fair'? Isn't any change you make to your page for SEO purposes an attempt to influence rankings? Why is one method less pure than another when we're all just trying to get our pages to rank higher? Further, if an engine is ranking a bunch of irrelevant sites above mine, what's so wrong about using any method at my disposal to get my relevant site ranked above them? Doing so doesn't benefit just me, it benefits the searchers because it gives them what they're searching for. And it also benefits the engine, because searchers will think better of the engine for giving them more relevant results than it would have otherwise."
Adding to the controversy about Black Hat SEO is the fact Google does allow a certain select few sites to operate contrary to its own stated policies. Danny Sullivan complains that Google's cloaking policy is inconsistent (more on cloaking below), and that the policy wording should be updated. He further notes that the reason Google allows some cloaking is that it improves searching rather than hinders it, and that it's therefore inappropriate to think of cloaking as synonymous with "bad".
Whether you think Black Hat SEO is bad or not you should avoid it anway, because it can get you banned from the search engines, or at least reduce your ranking. Google has been known to remove sites it felt weren't playing fair. Granted, this isn't likely, but why take that risk? Also, much Black Hat SEO involves some fairly technical work. If this article is your introduction to SEO, you likely don't have the skills to be a successful Black Hatter anyway -- at least one who doesn't get caught.