XML structure is made of number of components, which are listed below.

List of XML document's structural elements
Name Description
XML Declaration Defines XML version.
Document Type Declaration Defines rules to be followed.
Comment Defines content area for comments.
Processing Instructions Instructions to the application.
Element Defines content area between element tags.
Attribute Defines content area for additional information about elements.
CDATA section Defines markup characters.
Content Defines the content itself.
Entity Defines any storage unit that holds data.

To obtain more detailed information regarding the structure in XML, follow this link:

http://www.brenkoweb.com/tutorials/xml/xml-structure

Subsets of XML DTD
Subset Description
Internal DTD Defines DTD internally.
External DTD Defines DTD externally.
Private specification Defines private declaration.
Public specification Defines public declaration.

To obtain more detailed information regarding the document declaration in XML, follow this link:

http://www.brenkoweb.com/tutorials/xml/xml-document-type-definition