I understand search engine optimization had to do with search engines, and social media optimization has to do with sites that invite the social media aspect to them.

There is not much clarity on the other terms. Internet or/and (as the case maybe) Digital Marketing. Is this something different?

Facebook is a social site, so this would go into social media optimization. However, when you put adds on facebook, this would be known as paid advertising, which is search engine optimization. So it's like one is the other under different circumstances.

I check a number of websites out, and they all list things differently. Some of the things inter-twine into each other, so it's hard to know what belongs to what. Is there any clear structure to what is what?