Online SEO is somewhat of a redundant phrase, due to the fact that SEO can only take place online and is non-existent offline. SEO deals with the search engines and optimizing a website for the best advantage and placement in any or all of the major search engines, including Google, Bing, and Yahoo. There are other search engines in other countries as well, that are well known in their respective areas.
Optimizing a website includes many factors, starting with the site itself. It includes the content, how it is displayed, where the site links to, and which sites link back to it. It can be complex or simple, but to be effective, it has to be by the rules.