XML

XML, the Extensible Markup Language, is a W3C standard for encoding structured documents and data in a format that is both human-readable and machine-parseable. It uses nested, tagged elements and attributes to represent hierarchical data, and it is validated against schemas like XML Schema (XSD) or DTDs. For much of the early web-services era, XML was the default wire format for data interchange.

• Tags and attributes - Data is expressed as nested elements with attributes, giving documents an explicit, self-describing tree structure.
• Schema validation - XML Schema and DTDs let you define and enforce the shape of a document, which is why it became the format of record for contracts and filings.
• A whole ecosystem - XPath for querying, XSLT for transformation, and namespaces for mixing vocabularies grew up around it.
• The foundation of SOAP - SOAP web services and their WSDL contracts are XML end to end, tying XML to a generation of enterprise integration.

XML still runs a huge share of the systems I map, even as JSON has taken over most new API design. In practice it shows up in SOAP services, in enterprise B2B and government document exchange, and in configuration and publishing pipelines that predate the JSON and YAML era. For governance and the agentic turn, XML is frequently the format an older system speaks, so modern APIs and agents have to read, transform, and translate it rather than assume everything arrives as JSON.