The video below is a great attempt at explaining it in layman friendly terms, but I doubt that most people, especially the majority of those whose job touches the web in one form or another, will really grok it. I blame Word Processors, which taught a generation of folks that content and formatting are basically one and the same. If more people understood how they are separable, you’d see more users using Microsfot Word’s Styles to manage the appearance of their documents, instead of hard-coding bold and font sizes to what they want to look like a title.