What are commonly used namespaces in .NET for XML ?

The commonly used namespaces in .NET for XML are :

1. System.Xml
2. System.Xml.Schema
3. System.Xml.XPath
4. System.Xml.Xsl

What is well formed XML document ?

Well formed XML document follows below rules :

1. Every start tag has end tag
2. XML document has root element enclosing all other elements
3. Opening and closing tag must match  and must be written in same case
4. Attribute values should be enclosed with double quotation marks.
5. XML tags must be properly nested

Difference between XML DOM and Simple API for XML

1. XML DOM loads the entire xml document into memory whereas Simple API for XML ( SAX ) does not load entire xml document into memory
2. XML DOM is used when you want to modify xml document and perform complex searches whereas SAX is not used when you want to modify xml document and perform complex searches