In our SEO business the rights and wrongs of ethics are referred to as white and black hat techniques respectively. A white hat SEO is the 'good boy' who does everything in the prescribed manner, while black hat techniques are adopted by the mischievous elements of the industry.
White hat SEO techniques are all those that books teach you. It involves increasing the keywords population in your content in a natural manner, and not stuffing it. It follows natural link exchange between your site and others. There is lots of emphasis on generating fresh content regularly to keep the site updated. Emphasis is laid on creating keyword friendly Meta tags & descriptions, titles, headers etc. All these techniques are recognized, rather appreciated by search engines.
The purpose behind using black hat search engine optimization is simply to get quick results with minimum efforts. Basically, it's a shortcut to success, but the repercussions are there to be faced for using this technique.
One of the most commonly used black hat SEO technique is creating hidden text. Hidden text is basically stuffing the content with repeated keywords but making it invisible to the visitors by writing the text in same color as the background. Other method is duplicating content of other sites on your own site. Cloaking which means having different versions of the same page for visitors and search engines.
These black hat techniques are no doubt a big temptation, just like any other forbidden thing, but the risk attached are high. You might and most likely will achieve high search engine rankings immediately but this ranking can never stay. You run the risk of a major ranking fall for a long duration and in many cases the site gets spammed all together.
Search engines are aware of these practices more than you and me. They constantly upgrade their algorithms to counter and detect these malpractices. So, a black hat technique that worked 6 months ago is not likely to work now. In short, the success, no matter what you do is limited.
Therefore, when you choose which color SEO hat you want to wear - Black or white, remember that black hat might get you an instant high but it will lose the fizz at equally fast pace. White hat will be gradual but long lasting. Even in this case, slow and steady wins the race.