What is XML?

Extensible Markup Language (XML) defines a set of rules for encoding documents in a format that is both human-readable and machine-readable.

Each element markup construct begins with “< >“ and ends with “</>"

start-tags: <section>

end-tags: </section>

A Document Type Definition (DTD) defines the legal building blocks of
an XML document. It defines the document structure with a list of legal
elements and attributes.

An XML Schema is an XML-based alternative to DTD. An XML schema
describes the structure of an XML document. The XML Schema language is
also referred to as XML Schema Definition (XSD).