System.XML.XMLDocument

Introduced in .NET 1.1

This allows you to browse the nodes in any direction, insert and remove nodes and change the structure on the fly.
Loads the entire XMLDocument into memory.
You can have problems when using this method because the data model is different to XSLT and Xpath

This is based on the Document Object Model (www.w3c.org/DOM)
Access to data in a non-sequential way

Load - loads from a string, stream, text reader, xml reader
LoadXML - loads from a string

If you want a more memory-efficient alternative and you can process the XML bit by bit, consider using the XMLReader and XMLWriter classes.

The XMLDocument class is an XML parser which stores the contents in memory

An XPathNavigator can be created over the content to provide easy access to nodes and values

This class implements W3C level 2 XML DOM core functionality.

Dim objXMLDocument As System.XML.XMLDocument 
WIth objXMLDocument 
   .CreateXMLDeclaration 
   .InsertBefore 
   .InsertAfter 
   .CreateElement 
End With 

XMLAttribute

An element can have one or more attributes
Attributes can be used to control the XML serialization on an object.

<System.Xml.XmlAttribute("name")> 

System.XML.XMLDataDocument

This is now obselete
This class is inherited from XMLDocument and exposes the XML content as an ADO.NET DataSet instance.
This should only be used when you need to access your data as a DataSet.

It was used to allow structured data to be stored, retrieved and manipulated strough a relational DataSet.

System.XML Namespace

This namespace contains a large number of classes for reading, processing and handling XML