XML Review
In a nutshell, XML is a format for storing structured data. Although it looks a lot like HTML, XML is much more strict with quotes, properly terminated tags, and other such details. XML does not define tag names, so document authors must invent their own set of tags or look towards a standards organization that defines a suitable XML markup language . A markup language is essentially a set of custom tags with semantic meaning behind each tag; XSLT is one such markup language, since it is expressed using XML syntax.
The terms
element
and
tag
are often used interchangeably, and both are used in this book.
Speaking from a more technical viewpoint, element refers to the
concept being modeled, while tag refers to the actual markup that
appears in the XML document. So <account> is
a tag that represents an account element in a
computer program.
SGML, XML, and Markup Languages
Standard Generalized Markup Language (SGML) forms the basis for HTML, XHTML, XML, and XSLT, but in very different ways for each. Figure 1-2 illustrates the relationships between these technologies.

Figure 1-2. SGML heritage
SGML is a very sophisticated metalanguage designed for large and complex documentation. As a metalanguage, it defines syntax rules for tags but does not define any specific tags. HTML, on the other hand, is a specific markup language implemented using SGML. A markup language defines ...