XML
XML (Extensible Markup Language) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. It is widely used for representing structured data, configuration files, and data interchange between heterogeneous systems. XML documents consist of elements defined by tags and can be validated against schemas like DTD or XSD.
Developers should learn XML for scenarios requiring strict data validation, hierarchical data representation, or integration with legacy systems and enterprise applications. It is essential in domains like web services (SOAP), document storage (e.g., Microsoft Office files), and configuration management (e.g., Android layouts, Maven POM files). XML's self-descriptive nature makes it suitable for complex data structures where metadata is important.