XML or eXtended Markup Language, has been a buzz word for quite a long time. And with the proliferation of AJAX, it has seemed to gain more credibelity. More so, it has been pitching in the compatibility front.
XML, as stated above, stands for eXtended Markup Language. It is not a programming language, but a mark-up language. Even HTML has been wrongly considered as a programming language, while like XML it is a mark-up language. The history of XML, seems to be an evolution out of SGML - Standard Generalized Markup Language.
XML, like SGML, is not a markup language in itself, but a specification for defining a markup language. HTML is a markup language, where as SGML is not. SGML tells how to define HTML. Hence, HTML is application of SGML. Similarly, with the advent of XML we have found HTML evolving into XHTML. Here XHTML is a application of XML. Note that XHTML is just a subset of the many applications of XML.
Using a markup language - standardised markup language - one can use any system or application, and still be able to exchange document or more generally data with others who might be using a different system or application. If there is a standard markup for a document, then effectively, one should be able to open it in Open Office Word, Microsoft Word, or may be even my own home-brewed application. You are not bound by a proprietary format.
XML is not a magic wand that will let us all live happily ever after. It would still require different software vendors of a particular league of application to sit down together and work out a XML definition for their data. This has happened to some extent in terms of the browser players - by the introduction of XHTML. There has been a moderate standardisation in vector graphics file format. There are qutie a lot of application that support SVG - Scalable Vector Graphics - as a file format for vector graphics. SVG is a application of XML.
Some of the applications of XML are:
Some of the other things that are worth a look are WSDL and SMDL. WSDL or WebSerivce Description Language is a major key player already even though it has not yet become a W3C recommendation yet. SMDL, Standardised Music Description Language is for marking up musical notation. It is presently a ISO standard.