Anyone who's ever administered even the most modest database knows that we humans need to be protected from ourselves at every turn. If given the opportunity, we'll omit crucial information and include extraneous nonsense. That's why the XML creators, being benevolent and understanding of human frailty, included the Document Type Definition, or DTD. The DTD provides a way to make sure the XML is more or less like you want it.
It may just be because I've been listening to the audiobook presentation of The Bad Beginning, but I can hear Tim Curry reciting that passage perfectly inside my head.
What's next? "People who enjoy happy and useful tutorials -- the word tutorial here means 'series of web pages that teach you how to do something' -- should direct their browser to another page immediately. I wish I could tell you that this XML tutorial ends happily, by giving you all the information you need to create your required ColdFusion report that generates an RTF document via XML. As I say, I wish I could tell you that, but alas, I cannot, for that isn't what happens. This tutorial does not in fact teach you anything useful at all, and by the time you reach the end of it, you'll realize that the time you spent reading it is merely hours and hours of your life that you can never have back."