XDR was an early attempt by Microsoft to provide a more comprehensive standard than DTD.
This standard has pretty much been abandoned now in favor of XSD.
Tutorial 3 validating an xml document
formally describes what a given XML document contains, in the same way a database schema describes the data that can be contained in a database (table structure, data types).
An XML schema describes the coarse shape of the XML document, what fields an element can contain, which sub elements it can contain, and so forth.
It also can describe the values that can be placed into any element or attribute.
DTD was the first formalized standard, but is rarely used anymore.
Elements are the main building block of any XML document; they contain the data and determine the structure of the document.
An element can be defined within an XML Schema (XSD) as follows: An element definition within the XSD must have a name property; this is the name that will appear in the XML document.
The type property provides the description of what can be contained within the element when it appears in the XML document.
The XSD standard has evolved over a number of years, and is controlled by the W3C.
It is extremely comprehensive, and as a result has become rather complex.
For this reason, it is a good idea to make use of design tools when working with XSDs (See XML Studio, a FREE XSD development tool), also when working with XML documents programmatically XML Data Binding is a much easier way to manipulate your documents (a object-oriented approach; see Liquid XML Data Binding).
The remainder of this tutorial guides you through the basics of the XSD standard, things you should really know even if you're using a design tool like Liquid XML Studio.