XML (Extensible Markup Language) is a general-purpose markup language that describes data structure using tags. Like HTML, it derives from SGML, but its distinguishing feature is the ability to define custom tags.

XML has a rich ecosystem of related technologies including namespaces, schema validation (XSD), and transformation (XSLT). It is used in many domains including sitemap.xml, RSS feeds, SOAP APIs, and Office documents (OOXML). XML fundamentals books cover the basics comprehensively.

In recent years, JSON has been replacing XML as the response format for REST APIs, but XML remains the choice when document structure expressiveness and strict schema validation are required.

From a character count perspective, XML has many syntax elements like tags and attributes, tending to use more characters than JSON for representing the same data. Data format comparison books provide additional insights.