For that matter, what is XML??
You've probably seen XML, or at least another SGML-subset like HTML
They look like this:
<tag attribute="value" anotherAttribute="anotherValue">
Some content
<aNestedTag someAttribute="more space separated values" />
More Content
<!-- a comment -->
</tag>
It was created by the W3C, the folks who really standardized HTML,
invented CSS, RDF, SMIL, SVG, Atom (as in news feeds, not the
elementary particle) It's popular because it's simple, and there are
standardized ways for accessing it, and manipulating it (XPath,
XQuery, XSLT, DOM, etc.) It's main use is as a file for meaningful
data, separate from the program that manipulates or generates it. The
new Microsoft Word uses an XML vocabulary as their default file
format. Operating systems more and more seem to be full of it. I've
said enough.
see the W3C's page
http://www.w3.org/XML/
-Daniel Brumbaugh Keeney