One of the seminal papers on XML, by PhilipWadler and Jerome Simeon. http://homepages.inf.ed.ac.uk/wadler/papers/xml-essence/xml-essence.pdf

Because many people discuss XML related issues here without much awareness of what's in this paper is worth giving it a wiki name, a short summary and maybe some discussions.
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'''The main ideas from the paper.'''

What is XML ? XML is an "external" format for representing data. It is external in the sense that it is not particular to any programming language or software system. 

The paper asserts that 2 keys property are required of any such format :

* '''self describing'''. The xml document should be enough to unambiguously get the data.
* '''round tripping'''. Converting the data from internal representation in a software system (such as an object inside a Java application server, structures within C runtime, rows within Oracle tables, etc) to XML and back again, should yield the same data. In other words : 
   data == DecodeFromXML(EncodeToXML(data))

An example of a data format with these 2 properties is Lisp EssExpressions. Although XML comes close to having these properties it fails on both account. So According to Wadler, Simeon (and many many others) the essence of xml is this:
 '''the problem it solves is not hard and it does not solve the problem well.'''
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Hence, XmlIsaPoorCopyOfEssExpression and other related pages.