If you look for information about SEO in the Web you will have no problem to find an endless list of suggestions, tools and examples about the argument. But looking with more attention you will be soon able to divide into two main group the techniques indicated: a first one which consists in a group of more or less basic SEO techniques based in the way the genuine content, that your website is already made of, should be enhanced from the SEO point of view; and a second one which consist in using some advanced means to present in a different way the website, and the pages themselves, to the human users or to the search engines.
In the second case for instance are added pages with links to an important page, or it is added hidden text, full of keywords, to a single page; this content is not shown to the human users because it is meaningless but it can give a further boost to the rank position of the website in particular for very competitive search terms. The peculiarity of the this group of techniques is that, if a search engine spotted them, your website would lose position in the result list and in the worst case your website might be banned. It is true that also exaggerating in the application of the first group can be dangerous, but it is quite clear that the techniques of the second group are much more dangerous.
The techniques of the first group are usually called "ethical SEO" and the techniques of the second group SEO spam or black hat SEO techniques. Phil Craven , an influent SEO expert, in a series of articles points out his position. He claims that for the search engines SEO include only: writing copy, giving advice on site architecture and helping to find relevant directories to which a site can be submitted. It means that for search engines both the ethical and the unethical techniques are deprecated, that is the reason you have to carefully optimize also using ethical SEO. Anyway it also true that a search engine cannot show all the relevant information at the top of the ranking, SEO spam are powerful techniques to achieve that your website, which maybe is relevant as much as others, will be considered relevant enough by the search engine to appear at the top of the ranking.
Beside the ethical issues that everyone can deepen as much as he wants, it appears clear that as far as you have to optimize a website to a two word search sentence where there is not too much competition you can just use the ethical SEO, described in the chapter two of this tutorial, which is less dangerous, easier and faster. But whenever you will need to optimize a website for high competitive two word searches or, even worse, common one word searches you will need the SEO spam techniques, described in the chapter four.
I will also speak, in the chapter three, about the way a search engine sees your website, which affects the way you should develop it.
Finally I will analyse other general topics about optimization in particular circumstances.
If it is true that you have to carefully optimize to not be banned, it is also sure that not following the main SEO rules will lead to very poor ranking positions, but, to understand how you can optimize a website, it is before necessary to analyse how a search engine actually works.