More on XML
This was quite a short part on XML and there is a lot more on XML out there. I'm not that well educated in the field of XML (mainly because I don't plan on using it for anything that would need me to learn the intricate details of it). This parts was designed to show you one of the many roles of XML in ASP.NET. If you check out my munted calculator at http://authors.aspalliance.com/wisemonk/articles/samples/cal.asmx and try to add something, you will see that the result is an XML file.
If you want to learn more about XML then take a look at the official definition at http://www.w3.org/XML/ or get a less technical look at http://www.4guysfromrolla.com/webtech/xml.shtml.