SEO is one of internet marketing which full form is search engine optimization.  To boost company it's website in internet marketing they must do work through SEO which plays vital role to update company website in search engine ranking.  On site optimization and off site optimization are the major factor that effects ranking in the search engine.
As all we know content is the king of a site,but they should be unique one not copied from others and building quality back links will certainly boost the ranking of a sites. Besides them link wheeling,social bookmarking are other SEO technique

Laxman, you are  correct, but you know, these days Search Engine optimization has gone beyound what you listed above. And has become complete course of it own with differents parts.