XML- Defined

The main reasons for my choosing to examine XML, with Java as the development environment are:

I have never used XML.
I am very interested to know the differences in XML and other markup languages.
There seems to be a meteoric rise in the use of XML amongst programmers Java is way outside my comfort zone.

So the first thing to do is find a definition for XML, the following is the web definition from wikipedia :

(source: website http://en.wikipedia.org/wiki/XML)

"The Extensible Markup Language (XML) is a W3C-recommended general-purpose markup language that supports a wide variety of applications. XML is also designed to be reasonably human-legible, and to this end, terseness was not considered essential in its structure. XML is a simplified subset of SGML. Its primary purpose is to facilitate the sharing of data across different information systems, particularly systems connected via the Internet. Formally defined languages based on XML (such as RSS, and thousands of other examples) allow diverse software to reliably understand information formatted and passed in these languages."

That's all well and good, but for clarity and 'good investigative procedures' let me compare this to what the iTC594 course has to say on the subject.
Ken Eustace points out that:
(source: website http://ispg.csu.edu.au/subjects/itc382/resources/lectures2004/topic5#258,3,)

"XML is not a markup language like HTML, it is a language used to describe a markup language. The technical term for such a language is metalanguage
Using XML a developer can define markup languages which describe electronic circuitry, information for electronic data interchange, the files produced by Web servers, mechanical parts of aircraft and so on."

"A developer define a particular language using XML and a tool or utility then takes XML documents which contain text expressed in the language and carries out some process such as converting it to an MS Word document or into some other form."
(cited from Ince D, 2004 "Developing Distributed and E-commerce Applications", Chapter 8, pp 213)

And finally, lets see how the XML working group describe XML

"The Extensible Markup Language (XML) is a subset of SGML whose goal is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML. XML has been designed for ease of implementation and for interoperability with both SGML and HTML."
(source: website http://www.w3.org/TR/2006/REC-xml-20060816/#sec-terminology)

"XML is a new specialization that enables Web page designers to create their own, customized tags top provide functionality the is not available using the current markup language used for many Web applications."
(XML and Java Developing Web Applications: Hiroshi Maruyama, Ken Tamura, Naohiko Uramoto 3rd Ed.)