It can be tough explaining what Markdown is to a normal human being. I've struggled with describing the expanse that is MultiMarkdown. What's a header when you don't know what HTML is? No more explaining.
Introducing the Markdown Field Guide by Eddie Smith and David Sparks.
This is a wonderful tour of Markdown. It guides the reader from the basics up through the sophistication of MultiMarkdown. It is well organized, simple and beautiful.