Basically SEO can be separated into 2 types

Search Engine Optimization definition

Search engine optimization or SEO refers to improving a web sites' natural and organic ranking in the search engine results page. This more affordable method is often slow and new web sites may take up to a year to achieve decent rankings in Google. The methods of achieving high rankings are based on 2 main components, which follows,

On-page optimization Keyword density Title Tag optimization Keywords in the content Quality of original content Keywords in hyperlinks Keywords in headings and bold tags Keywords in URL Meta Tags (some engines) Sitemap Link structure "Indexability" of the web page Off-page optimization Amount of incoming links Quality of sites that link to yours Relevancy of sites that link to yours Keywords in the anchor text