SEO consists of many things — generally it involves the process of improving the content and link profile of a website with the goal of achieving visibility in the organic / natural results of search engines with a strong emphasis on Google and Bing/Yahoo. While most companies that provide SEO have an internal playbook of theories on how to improve rankings for a website, the truth is that no-one (excluding the engineers who manage the algorithms) knows exactly how a search engine will value a given website.
The process of SEO is never over. It involves trial and error, and continually reverse engineering a website based on what you know to have worked with other websites. The process rewards those who are good at asking “what ifs” and “what now,” while looking past the buzz of the most recent gimmick.