XML

In short: “Extensible Markup Language” — a text-based format related to HTML for structured data storage, with self-defined tags instead of fixed HTML tags.

In more detail: Unlike HTML (a fixed set of tags for the web), with XML you define your own tag names to fit your own data (e.g. <customer><name>...</name></customer>). Long the standard for configuration files, data exchange (SOAP APIs) and Office file formats (.docx, .xlsx are internally XML) — largely replaced for new APIs by the more compact JSON now.

In Depth

<customer id="42">
  <name>Anna Muster</name>
  <email>anna@example.com</email>
  <orders>
    <order id="1001" date="2026-03-15" />
  </orders>
</customer>

XML allows arbitrarily nested, self-describing structures with attributes (id="42") AND child elements — considerably stricter than HTML, which only knows a fixed, predefined set of tags with a fixed meaning. This strictness has a price: every opening tag needs a matching closing tag, which makes XML considerably more text-heavy than JSON (the same customer object would be shorter and without redundancy in JSON).

XML was the standard for structured data exchange for decades, especially in enterprise environments (SOAP web services, EDI in trade, many government interfaces), and remains partly mandatory there today, because established systems build on it. For new web APIs, JSON has largely become established, since it’s more compact and can be read natively in JavaScript — but XML lives on in many “invisible” roles: Microsoft Office files (.docx, .xlsx) are internally ZIP archives of XML files, SVG graphics are XML, and Android app layouts are traditionally described in XML. XML also offers, with XSD (XML Schema Definition), a standardised way to formally validate a document’s allowed structure — a feature that was only added to JSON later and less uniformly (JSON Schema).

See also: HTML, JSON