XML / XLS application

> Hello mailing list, I have a question about the use of XML / XLS > application, I am introducing myself in this technology, What is useful XML > / XLS for? , I need a basic example, basic application of it, because I > don't find the use of it. I know that XML is used to interchange data > between every system in standard way (Why XML is standard?, because I > understand that depend of the client browser they have a particular XML > parser to retrieve information of the XML document, if would have only one > standard XML parser, could be), and with XLS you transform the XML document > to HTML format, postscript format, and so on, but in order to transform the > XML document you need to know the syntax of the XSL and I think that its > another language that you need to know. Another doubt is if XML document > replace DB, because I read a book that has a methodology to transform DB > table into a XML document, and use a XML parser to retrieve the data, I read > another example that the script use a SQL statement like XML SQL Select, you > execute it, then the DBMS server give you the XML document, then XSL or XLST > convert the XML document, and finally you obtain the HTML in the client > browser, well, Thanks for any help, bye :).