Internet marketing, also known as SEO, is typically considered the most effective form of marketing, online or offline. That is because having Internet marketing performed correctly and ethically on one's Website allows the site to reach the most online visitors, across the most states and countries, across many search engines, for numerous keyword terms.

Other advantages include: being able to build brand recognition, a site can achieve first page keyword rankings across search engines like Google, MSN, Yahoo and others, eliminating costly PPC programs if a site is optimized naturally/organically in engines, and a site owner can see a fast ROI (return on investment).

Internet marketing is a general word used in marketing products and service online using valorous means and tools, SEO is just one aspect of internet marketing that falls under search engine marketing and can't be regarded as internet marketing as a whole  nor used interchangeable suggested by your post.

Internet marketing is  too broad to be called SEO. Because in internet marketing you have Article marketing, Affiliate marketing, PPC(pay per click), PPM(pay per impressions), PPL(pay per lead), PPA( pay per action), SMM( social media marketing), forum marketing, email marketing etc.