There's a Far Side cartoon by Gary Larson about an unusual chicken ranch. Instead of strutting around, pecking at seed, the chickens are all lying on the ground or draped over fences as if they were made of rubber. You see, it was a boneless chicken ranch.
Just as skeletons give us vertebrates shape and structure, markup does the same for text. Take out the markup and you have a mess of character data without any form. It would be very difficult to write a computer program that did anything useful with that content. Software relies on markup to label and delineate pieces of data, the way suitcases make it easy for you to carry clothes with you on a trip.
This chapter focuses on the details of XML markup. Here I will describe the fundamental building blocks of all XML-derived languages: elements, attributes, entities, processing instructions, and more. And I'll show you how they all fit together to make a well-formed XML document. Mastering these concepts is essential to understanding every other topic in the book, so read this chapter carefully.
All of the markup rules for XML are laid out in the W3C's technical recommendation for XML version 1.0 (http://www.w3.org/TR/2000/REC-xml-20001006). This is the second edition of the original which first appeared in 1998. You may also find Tim Bray's annotated, interactive version useful. Go and check it out at http://www.xml.com/axml/testaxml.htm.