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XML is a markup language same as HTML. XML tag you can define your self for storing data. XML store (describe) data, nothing more (either temporary or permanent). XML was designed for describing well formed data. Also XML have some strict rules. That rules follow every XML document.
1 HTML can be parsed as text/html and application/xhtml+xml, over and above what XML can be parsed as: application/xml.  XML and XHTML have to have a closing element for each open element. Depending on the element, HTML doesn't care whether an element has a closing tag or not.