An XML document has two sections, XML Prolog and XML Elements. Prolog is optional but if used must appear before the Elements. Prolog section can have XML Declaration and Document Type Declaration, both of which are optional.
The elements section of the document contains XML elements in the form of a tree structure. The first element in the elements section is called the root element. The root element will have child element(s) and the child element(s) will in turn have other child element(s). Every element except the root element will have exactly one parent.
Tree diagram of the movie catalog XML