This first chapter introduces you to XML. It explains in
general what XML is and how it is used. It shows you how
the different pieces of the XML equation fit together, and how
an XML document is created and delivered to readers.
What Is XML?
XML stands for Extensible Markup Language (often written as
eXtensibleMarkup Language to justify the acronym). XML is a
set of rules for defining semantic tags that break a document
into parts and identify the different parts of the document. It
is a meta-markup language that defines a syntax used to define
other domain-specific, semantic, structured markup languages.

XML Is a Meta-Markup Language
The first thing you need to understand about XML is that it
isn’t just another markup language like the Hypertext Markup
Language (HTML) or troff. These languages define a fixed set
of tags that describe a fixed number of elements. If the markup
language you use doesn’t contain the tag you need—you’re
out of luck. You can wait for the next version of the markup
language hoping that it includes the tag you need; but then
you’re really at the mercy of what the vendor chooses to
include.