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The Extensible Markup Language (XML) is a general-purpose specification for creating custom markup languages. It is classified as an extensible language because it allows its users to define their own elements. Its primary purpose is to help information systems share structured data, particularly via the Internet, and it is used both to encode documents and to serialize data. In the latter context, it is comparable with other text-based serialization languages such as JSON and YAML.

It started as a simplified subset of the Standard Generalized Markup Language (SGML), and is designed to be relatively human-legible. By adding semantic constraints, application languages can be implemented in XML. These include XHTML, RSS, MathML, GraphML, Scalable Vector Graphics, MusicXML, and thousands of others. Moreover, XML is sometimes used as the specification language for such application languages.

XML is recommended by the World Wide Web Consortium (W3C). It is a fee-free open standard. The recommendation specifies both the lexical grammar and the requirements for parsing

Well-formed and valid XML documents

There are two levels of correctness of an XML document: 

• Well-formed. A well-formed document conforms to all of XML's syntax rules. For example, if a start-tag appears without a corresponding end-tag, it is not well-formed. A document that is not well-formed is not considered to be XML; a conforming parser is not allowed to process it
• Valid. A valid document additionally conforms to some semantic rules. These rules are either user-defined, or included as an XML schema, especially DTD. For example, if a document contains an undefined element, then it is not valid; a validating parser is not allowed to process it.

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