I saw this explaination a couple of days ago in the help file for a free program I downloaded from Downloads.com. This program is fantastic, by the way, for anyone who wants to learn or use xml (xhtml).
Altova(.com) XMLSpy® 2004
The XML Specification
The W3C specification defines XML as a subset of SGML, so to properly understand XML, it is useful to take a closer look at SGML first.
SGML stands for Standard Generalized Markup Language, and was developed for large scale applications, aircraft maintenance or power plant documentation, and intended to be maintained over the long term.
The reason why XML seems to be so similar to HTML lies in the fact that HTML is defined as a subset of SGML. XML is actually a lot more similar to SGML than to HTML, because HTML is only one specific subset of SGML used to describe web pages.
As XML was created to simplify SGML, it is no wonder that the W3C has now decided to redefine HTML 4.0 as an XML application, thereby creating XHTML 1.0. But this shall be of no concern for us at the moment, because we are still faced with the fundamental question "What is XML?".
To answer this, let us define what XML is not:
· It is not a programming language.
· It is not the next generation of HTML.
· It is not a database.
· It is not specific to any horizontal or vertical market.
· It is not the solution to all your problems, but it can be a very powerful tool in building such a solution.
XML is a clearly defined way to structure, describe, and interchange data.
Data in this context really means every conceivable kind of data! You can use XML for such diverse things as describing mathematical formulas, chemical compounds, astronomical information, financial derivatives, architectural blueprints, annotating Shakespearean plays, collecting Buddhist wisdoms, or voice-processing in telephone systems!