XML is if you look at it just a text file which is a bit fancy then a comma separated file.
Its power is in the standardds that it follows and the DTD system that you can create.
XML Itself does nothing ,but its a extremely powerfull tool when combined with any of the programming languages..such as java...etc
I'd say start browsing the www.xml.org and go from there.
XML is just a way of storing information. You can use other languages to display it.
XML does not contain any information on what the data should look like. It only says what that data is.
In XML-you define your own tags.
An example is
Its just data. Then you would use another language(preferably XSLT to say...everything in the <to> tag should be centered and bolded, everything in <from> should be a mailto: link, the subject should be italicized, and the message should be displayed.