SEO - Search Engine Optimisation is a technique to improve the ranking of your website. Its a generic(unpaid)marketing discipline focused on your Website. This technique is useful and stable. All search engines like google, yahoo, bing, etc. have their own indexing system. In SEO we work with content and images on our website, this will help us to index our URL with these search engines. We use some more techniques for this like submitting our URL in directories.
SEO has two types one is a black hat technique and another is white hat. This white hat technique is always preferred to improve ranking. This technique is suggested because it improves our ranking gradually, but surely. On another hand, the black hat technique is not good, because it has some issues which are not suggested by search engines and some times this leads to the downfall of your ranking.
The SEO focuses on optimizing your content only for the search engines, which are not good. The best way to optimize your site is to update the content and submission in relative directories and try to avail of the backlinks. I suggest that you go with google norms and grow your site in mannered fashion. Since there are lots of ways to bend and break the rules to get your sites to rank high but try to avoid the false mode. Ultimately, this approach results in spammy, crappy pages that often get banned very fast.
White hat SEO, a genuine mode, this is the way to build a sustainable online existence. This method of SEO helps you to focus on your human audience as well not only the search engines. The best promotion is to prove yourself to your target audience.