XML Markup and Core Concepts
http://www.oreilly.com/catalog/learnxml2/chapter/ch02.pdf

Ray, Erik T.
O'Reilly and Associates
2003

Abstract:

This chapter focuses on the details of XML markup. It 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.