XML

XML, or Extensible Markup Language, describes data using nested tags that you define yourself, much like HTML but without a fixed vocabulary. An XML document wraps content in opening and closing tags, can attach attributes to elements, and must be well-formed: every tag closes, elements nest properly, and there is a single root. XML predates JSON as an interchange format and remains common in configuration files, document formats, and enterprise systems. It is more verbose than JSON but supports features such as namespaces, schemas for validation, and mixed content, which is why it persists where documents are complex.