xml what is the purpose

Check out http://www.w3schools.com/xml/xml_whatis.asp[^].

XML came about in the era of HTML wars where everyone was defining their own tags, so the standards comitee gave up and said here you go now you can define your own schema and tags to your hearts content.

After the dust settled on the browser wars, people saw that XML makes a lot of sense for data transfer between different systems and servers.

So XML and xsl lost favour as HTML generators and exploded as human and machine readable data transfer format.

Very basically: this is because HTML sucks. It's its "relaxed" HTML syntax and a lot of uncertainties in interpretation of tags created the situation when HTML Web pages are rendered with so many difference by any pair of different layout engines, see http://en.wikipedia.org/wiki/Comparison_of_web_browser_engines[^].

So, it was natural that people created the language which would suck much less and gets more universal application. When I say "much less" I mean it: it still sucks. I feel it well because years ago when good parsers were not yet available I happened to develop some XML processing and knew a good number of inconsistencies, especially in the pretty lousy text of the standard. This is well illustrated by the Annotated XML Specification by Tim Bray, see http://www.xml.com/axml/testaxml.htm[^].

—SA