XML (eXtensible Markup Language) is a markup language similar to HTML designed to store and transport data recommended by the World Wide Web Consortium (W3C).

Example of XML document
<?xml version="1.0" encoding="UTF-8"?>
  <item category="validator">
    <title lang="en">XML Validator</title>
    <author>Code Beautify</author>
  <item category="beautifier">
    <title lang="en">XML Beautifier</title>
    <author>Code Beautify</author>

XML Tree Structure
Xml document is constructed by element trees, starting with a root element and branches to child elements, therefore called tree structure.
To understand the relationships between elements in a XML document, there are 3 terms: parent, child, and sibling. Parent elements have children elements. Children elements have parent and siblings are children elements on the same level, like sisters and brothers.

Each parent element can have one or more children, and each child element can have one or more child elements, which they become parent element themselves. For example, if we take the above XML document, the items is the parent of the item element. The item element has two children, title and author, therefore he is the parent, but also a child of items element.