SEO basically focuses on how how one optimize their visibility by various search engines.This can be achieved through key words,up to date content,link building,bookmarking etc
In the SEO everything is most important. If we complete proper way On-Page like effective title and description creation. its help us most to get good ranking on serp. In the of-page we need to do directory submission, article writing and submission, blog comment, blog creation and promotion, rss submission, forum posting, link exchange with niche sites.
All are main seo, but mainly you can not leave Directory, Article and bookmarking. Also having some regular changes on page.